Short answer
HubSpot's KB: the Super Admin running it must still have an email on the current domain. Go to Users, Actions > Change user email domains. With more than one Super Admin, a second Super Admin approves before you confirm.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Line up the second approver before starting.
3. Related
Changing one login email and rebrand checklist.