The client portal login belongs to your client, and can be used by them to access their managed accounts across all of their properties and management companies.

A portal login account change is done by your client, through their portal. Instruct them to log in to their portal using the old email address, selecting the Account Config tab and changing the email/username at the bottom of the page. 

When you change a client's email address in their profile, this will change how you communicate with them, but they will still log in to their portals with the email address they determined for their portal access.