Changing your client's email address in Link will cause your client to lose access to all historical data and documents although it wouldn't affect your access to what's already in Intuit Link.

If your client would like to change the email address, there's a complicated process you and your client will need to go through to preserve access to the data and documents that are already in Intuit Link.  See this discussion for the steps to follow:

https://proconnect.intuit.com/community/lacerte-tax-discussions/discussion/re-client-login/01/115565...