My understanding is that you cannot change the taxpayer's user ID (or email address) for the taxpayer's Link account, which you do not own or control.

All you have in Proseries is only an email address that points to a specific Intuit account (the taxpayer sets up separately with Intuit, not only for Link but also other Intuit products) the collaboration tool should be "linked" for the engagement.  If the taxpayer wants to use a new email address but preserve all the contents in the existing account, I think the only way is for the taxpayer to contact Intuit directly so that they can change the user ID for the account.