I'm with dom on this,
The DLC will update only the things its told to update so if you change the appearance of a player with a pre-set face for example, once you install the DLC the pre-set face will NOT reappear and you will still have the face you made yourself.
This goes for the created players, so if you base copy they will remain after installing the DLC, that's why we have the base copy before DLC threads!
If the question is in regards to importing teams made by other people i.e. importing a fully created team over a fake team, I have no idea how this works, but trust me, on day one I will be trying to figure it out!