Hi
In my opinion, it's sure all profiles will be used for newly created users. For the old ones, you need some trigger to update the profile.
I suggest using the Membership.GetAllUsers method to retrieve all users. Then, I'd retrieve the profile for each user and take the old collection and bind them to your new profiles. Hope it helps.
Best Regards
XiaoYong Dai
Microsoft Online Community Support
Please remember to click “Mark as Answer” on the post that helps you, and to click “Unmark as Answer” if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread.