FindAndUpdate How to check if document was really updated

Imagine the following model:

var Office = 
{
    id: 1,
    name: "My Office",
    branches: 
    [
      {
        adddress: "Some street, that avenue",
        isPrincipal: true,
      },
      {
        adddress: "Another address",
        isPrincipal: false,
      },      
    ]
}

I'd like to remove a branch, but we can't let the user remove the principal branch from an office. So here's my function:

remove: function(body)
{
  return new Promise(function(resolve, reject)
  {
    return Office.findByIdAndUpdate(1, { $pull: {'branches': {_id: body.branch.id}}}, { new: true })
    .then(function(updatedOffice){
      resolve(updatedOffice)
    })
    .catch(function(error){
      reject(error);
    });

  })
}    

I have some doubts here:

1. As you can see I haven't included the another WHERE on the isPrincipal property, this is because I don't know how can I determine whether the office object got actually changed. Because the object will alway be retrieved but... How can I know this for sure?
2. Is FindByIdAndUpdate the best approach considering we can't let a user delete the principal branch, and if he's trying to do so, we have to show a warning.