Hello @WarwickM ,
Have you checked to make sure the "new" email address isn't unsubscribed (or among previously-deleted unsubs)?
Additionally, has this contact's current email been unsubscribed and resubscribed before? If so, the permissions for their current email address may be permanently set as a Compliance element.
Your best bet would be to use the copy-paste multiple method of uploading new contacts, and simply add that "new" email to the current list selection to make a new contact (adding single may result in the same error messaging).
If the "new" email is already unsubscribed or continues to have issues with being added, I'd recommend reaching out to our Review teams so they can do an in-depth dive into what's causing you issues with switching these email addresses.