Change fields with an enrollment key

I would like the ability to overwrite any field set for existing users with an enrollment key.

For example: An enrollment key is configured, and that key is set to place the user in department 'X'.

If a new user uses the key - they are placed in department 'X'

If an existing user uses the key who are already in department 'Y', they will NOT be moved to department 'X' upon using the key.

I want the ability for the enrollment key to be able to change any field, either standard ones, or custom fields. Even better if there was a tick box beside it to say overwrite yes / no, like in my mockup that would allow total flexibility.

Thanks