Sorry stupid question- how do I change my email address (for login)?
You should use a new email to register a new account.
There is currently no ability to change your email address (edit: you can view it by going to the "Support Us" page in the footer and picking membership/donate, but you can't change it from there). There doesn't appear to be anything in the codebase to handle changing it once the onboarding is complete. Some 3,500 temporary mail domains started being blocked on Thursday as well, so hopefully anyone who signed up with one had alternative authentication methods set up.
Edit: opened https://github.com/orgs/civitai/discussions/573 regarding this.