There is no need to migrate anything. All accounts remain personal accounts; all that changes is their file storage quota. While an account is part of a Zotero Lab subscription, it has unlimited file storage. If the account has it own personal file storage subscription, the timer on that is paused until either the Zotero Lab subscription expires or the account is removed from the Lab membership list.

Zotero for Institutions works the same way except that instead of you making a list of accounts that are part of the Lab, the unlimited storage is granted to all accounts that have an email address with the institution domain (eg, *@example.edu). Users can add multiple email addresses to their accounts, so they can add their institution email if they made their account with another email address.