This should not be the case, we primarily key on the SSO id.
You can use the sync sso endpoint to sync the emails on Discourse side if you want to do so proactively but changing emails in your system should always work.
The one exception is
Discourse thinks SSO id is 5 email is bob@jane.com
You log in with SSO id of 6 email is bob@jane.com
This is not resolvable so you will need to manually intervene. (sync sso id 6 first)