I didn't find any way to update existing app (.Net standard 4.6) to support new identity structure. So here is what I did to resolve this issue.
- As #Jeremy suggested in comments, I configured my .Net core Identity models to have different names (other then by default, so it does not have any conflict with existing tables) https://docs.microsoft.com/en-us/aspnet/core/security/authentication/customize-identity-model?view=aspnetcore-6.0#change-tablecolumn-names-and-facets (keep order of model configuration as it is as given in thread. I got issues when I kept my
base.OnModelCreating(builder); below builder configuration.)
- I have found a script to create identity tables schema and move records. see below screenshot. _New represents the .Net core identity tables scehma. I have attached that script here with and I have run this script in common database.

- So now I have common database with both old and new identity tables. old application is still pointing and working well with old tables which .Net core identity framework is poiting to new database.
- Since I have moved all the data and records from old tables to new table, I can access them in new idenity app too.
- Till now I have no need to run migration in new app, so I disabled it. but there was concern of what if I run migration and how will it affect existing database. so I did some R&D and found that we can create migration and before running that migration, we can just comment out the code inside the up and down method. after that we can run this migration and update database. basically empty migration to keep in sync with existing database. (haven't checked this working but this was my idea if I needed it)
Here is the sql Script that I used.
STEP 1 : Change name of existing tables
EXEC sp_rename 'AspNetRoles', 'AspNetRoles_old';
EXEC sp_rename 'AspNetUserClaims', 'AspNetUserClaims_old';
EXEC sp_rename 'AspNetUserLogins', 'AspNetUserLogins_old';
EXEC sp_rename 'AspNetUserRoles', 'AspNetUserRoles_old';
EXEC sp_rename 'AspNetUsers', 'AspNetUsers_old';
STEP 2 : Create ASP.NET Core Identity tables
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[AspNetRoleClaims](
[Id] [int] IDENTITY(1,1) NOT NULL,
[RoleId] [nvarchar](450) NOT NULL,
[ClaimType] [nvarchar](max) NULL,
[ClaimValue] [nvarchar](max) NULL,
CONSTRAINT [PK_AspNetRoleClaims] PRIMARY KEY CLUSTERED
(
[Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[AspNetRoles](
[Id] [nvarchar](450) NOT NULL,
[Name] [nvarchar](256) NULL,
[NormalizedName] [nvarchar](256) NULL,
[ConcurrencyStamp] [nvarchar](max) NULL,
CONSTRAINT [PK_AspNetRoles] PRIMARY KEY CLUSTERED
(
[Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[AspNetUserClaims](
[Id] [int] IDENTITY(1,1) NOT NULL,
[UserId] [nvarchar](450) NOT NULL,
[ClaimType] [nvarchar](max) NULL,
[ClaimValue] [nvarchar](max) NULL,
CONSTRAINT [PK_AspNetUserClaims] PRIMARY KEY CLUSTERED
(
[Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[AspNetUserLogins](
[LoginProvider] [nvarchar](128) NOT NULL,
[ProviderKey] [nvarchar](128) NOT NULL,
[ProviderDisplayName] [nvarchar](max) NULL,
[UserId] [nvarchar](450) NOT NULL,
CONSTRAINT [PK_AspNetUserLogins] PRIMARY KEY CLUSTERED
(
[LoginProvider] ASC,
[ProviderKey] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[AspNetUserRoles](
[UserId] [nvarchar](450) NOT NULL,
[RoleId] [nvarchar](450) NOT NULL,
CONSTRAINT [PK_AspNetUserRoles] PRIMARY KEY CLUSTERED
(
[UserId] ASC,
[RoleId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[AspNetUsers](
[Id] [nvarchar](450) NOT NULL,
[UserName] [nvarchar](256) NULL,
[NormalizedUserName] [nvarchar](256) NULL,
[Email] [nvarchar](256) NULL,
[NormalizedEmail] [nvarchar](256) NULL,
[EmailConfirmed] [bit] NOT NULL,
[PasswordHash] [nvarchar](max) NULL,
[SecurityStamp] [nvarchar](max) NULL,
[ConcurrencyStamp] [nvarchar](max) NULL,
[PhoneNumber] [nvarchar](max) NULL,
[PhoneNumberConfirmed] [bit] NOT NULL,
[TwoFactorEnabled] [bit] NOT NULL,
[LockoutEnd] [datetimeoffset](7) NULL,
[LockoutEnabled] [bit] NOT NULL,
[AccessFailedCount] [int] NOT NULL,
CONSTRAINT [PK_AspNetUsers] PRIMARY KEY CLUSTERED
(
[Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[AspNetUserTokens](
[UserId] [nvarchar](450) NOT NULL,
[LoginProvider] [nvarchar](128) NOT NULL,
[Name] [nvarchar](128) NOT NULL,
[Value] [nvarchar](max) NULL,
CONSTRAINT [PK_AspNetUserTokens] PRIMARY KEY CLUSTERED
(
[UserId] ASC,
[LoginProvider] ASC,
[Name] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
GO
SET ANSI_PADDING ON
GO
CREATE NONCLUSTERED INDEX [IX_AspNetRoleClaims_RoleId] ON [dbo].[AspNetRoleClaims]
(
[RoleId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO
SET ANSI_PADDING ON
GO
CREATE UNIQUE NONCLUSTERED INDEX [RoleNameIndex] ON [dbo].[AspNetRoles]
(
[NormalizedName] ASC
)
WHERE ([NormalizedName] IS NOT NULL)
WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO
SET ANSI_PADDING ON
GO
CREATE NONCLUSTERED INDEX [IX_AspNetUserClaims_UserId] ON [dbo].[AspNetUserClaims]
(
[UserId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO
SET ANSI_PADDING ON
GO
CREATE NONCLUSTERED INDEX [IX_AspNetUserLogins_UserId] ON [dbo].[AspNetUserLogins]
(
[UserId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO
SET ANSI_PADDING ON
GO
CREATE NONCLUSTERED INDEX [IX_AspNetUserRoles_RoleId] ON [dbo].[AspNetUserRoles]
(
[RoleId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO
SET ANSI_PADDING ON
GO
CREATE NONCLUSTERED INDEX [EmailIndex] ON [dbo].[AspNetUsers]
(
[NormalizedEmail] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO
SET ANSI_PADDING ON
GO
CREATE UNIQUE NONCLUSTERED INDEX [UserNameIndex] ON [dbo].[AspNetUsers]
(
[NormalizedUserName] ASC
)
WHERE ([NormalizedUserName] IS NOT NULL)
WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO
ALTER TABLE [dbo].[AspNetRoleClaims] WITH CHECK ADD CONSTRAINT [FK_AspNetRoleClaims_AspNetRoles_RoleId] FOREIGN KEY([RoleId])
REFERENCES [dbo].[AspNetRoles] ([Id])
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[AspNetRoleClaims] CHECK CONSTRAINT [FK_AspNetRoleClaims_AspNetRoles_RoleId]
GO
ALTER TABLE [dbo].[AspNetUserClaims] WITH CHECK ADD CONSTRAINT [FK_AspNetUserClaims_AspNetUsers_UserId] FOREIGN KEY([UserId])
REFERENCES [dbo].[AspNetUsers] ([Id])
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[AspNetUserClaims] CHECK CONSTRAINT [FK_AspNetUserClaims_AspNetUsers_UserId]
GO
ALTER TABLE [dbo].[AspNetUserLogins] WITH CHECK ADD CONSTRAINT [FK_AspNetUserLogins_AspNetUsers_UserId] FOREIGN KEY([UserId])
REFERENCES [dbo].[AspNetUsers] ([Id])
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[AspNetUserLogins] CHECK CONSTRAINT [FK_AspNetUserLogins_AspNetUsers_UserId]
GO
ALTER TABLE [dbo].[AspNetUserRoles] WITH CHECK ADD CONSTRAINT [FK_AspNetUserRoles_AspNetRoles_RoleId] FOREIGN KEY([RoleId])
REFERENCES [dbo].[AspNetRoles] ([Id])
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[AspNetUserRoles] CHECK CONSTRAINT [FK_AspNetUserRoles_AspNetRoles_RoleId]
GO
ALTER TABLE [dbo].[AspNetUserRoles] WITH CHECK ADD CONSTRAINT [FK_AspNetUserRoles_AspNetUsers_UserId] FOREIGN KEY([UserId])
REFERENCES [dbo].[AspNetUsers] ([Id])
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[AspNetUserRoles] CHECK CONSTRAINT [FK_AspNetUserRoles_AspNetUsers_UserId]
GO
ALTER TABLE [dbo].[AspNetUserTokens] WITH CHECK ADD CONSTRAINT [FK_AspNetUserTokens_AspNetUsers_UserId] FOREIGN KEY([UserId])
REFERENCES [dbo].[AspNetUsers] ([Id])
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[AspNetUserTokens] CHECK CONSTRAINT [FK_AspNetUserTokens_AspNetUsers_UserId]
GO
STEP 3 : Migrate data from old tables (ASP.NET Identity) to new tables (ASP.NET Core Identity)
INSERT INTO AspNetRoles ([Id], [Name], [NormalizedName], [ConcurrencyStamp])
SELECT [Id], [Name], UPPER([Name]), LOWER(NEWID()) FROM Objectra_Development.dbo.AspNetRoles
INSERT INTO AspNetUsers ([Id], [UserName], [NormalizedUserName], [Email], [NormalizedEmail], [EmailConfirmed], [PasswordHash], [SecurityStamp], [ConcurrencyStamp], [PhoneNumber], [PhoneNumberConfirmed], [TwoFactorEnabled], [LockoutEnd], [LockoutEnabled], [AccessFailedCount],
AssignObjects, AccessOwnObjectsOnly, UserObjectId, UserObjectDefd, GoogleAuth, LastPasswordResetDate, FriendlyName, AccessibleObjectDefinitions)
SELECT [Id], [UserName], UPPER([UserName]), [Email], UPPER([Email]), [EmailConfirmed], [PasswordHash], [SecurityStamp], LOWER(NEWID()), [PhoneNumber], [PhoneNumberConfirmed], 0, null, 1, 0,
AssignObjects, AccessOwnObjectsOnly, UserObjectId, UserObjectDefd, GoogleAuth, LastPasswordResetDate, FriendlyName, AccessibleObjectDefinitions
FROM Objectra_Development.dbo.AspNetUsers;
INSERT INTO AspNetUserRoles ([UserId], [RoleId])
SELECT [UserId], [RoleId]
FROM Objectra_Development.dbo.AspNetUserRoles;