Apple ID email conflict?

I've recently decided to go back on my Gmail account, so want to change my Apple ID login email to example@gmail.com. However, example@gmail.com is already associated with an old Apple ID of mine I no longer maintain, so I went into that account and changed the login to oldAppleID@example.com. Upon trying to change the login email of my primary Apple ID, I'm still met with the error message "this email has already been associated with another Apple ID" — even though Apple doesn't recognize that email as an Apple ID when trying to log back in with that same email. Is this a database issue on Apple's end? When can I expect the email to be unassociated?