Add member to existing struct without breaking legacy code

There is the following definition in some legacy code that I am working with.

struct VlanData
{
    uint16_t mEtherType;
    uint16_t mVlanId;
};

I want to add a new member to this struct:

struct VlanData
{
    uint16_t mEtherType;
    uint16_t mVlanId;
    uint8_t mVlanPriority; // New member
};

However, use of VlanData has been fairly inconsistent through the legacy code.

Not initialized on construction:

VlanData myVlans;
myVlans.mEtherType = 0x8100;
myVlans.mVlanId = 100;

Value initialized:

VlanData myVlans = { 0x8100, 100 };

What I want to do is come up with a safe way to ensure that `mVlanPriority' is automatically set to 0 in legacy code without updating a lot of code.

I understand I could modify the legacy code to value-initialize all members like so:

VlanData myVlans = {};

But I don't want to have to update all of this code. I believe that creating a default constructor like this would help:

VlanData()
: mEtherType(0),
  mVlanId(0),
  mVlanPriority(0)
{}

But this would also destroy the POD-ness of the struct.

So I guess I have several questions:

1. Is there a safe way I can ensure that mVlanPriority is set to 0 in legacy code without updating the legacy code?
2. What use of the class would break if this was no longer a POD type?