Cannot temporarily change current Apple ID to old one

My iPad Air is still running iOS v8.1.2. Can't update because overtime it asks for password of my previous Apple ID, which is an email address that was already discontinued.

I've tried to follow instructions to temporarily change my current Apple ID to the previous one, in order to sign-out iCloud on my iPad. The issue is that when I try to change the Apple ID it requires a validation code that was sent to the discontinued e-mail address!!! How do I work around this?

Thanks!