Can’t change Apple ID email address: error - email address not available

I’ve been using a university email address as my primary Apple ID address for about 10 years. Due to a change in policy with the university, alumni can no longer use their email addresses. I had a secondary email set up as my backup email for Apple ID and need to change this secondary email address to my primary. I’ve removed the secondary email address from my Apple ID account, but I still can’t change that address to the primary. I keep getting an error saying that the email address isn’t available. I removed the secondary email at least a month ago. I never used this email address with any other Apple IDs/Apple services.

Why am I getting an error saying my own email address isn’t available and how can I change this address to my primary email address for my Apple ID? Any tips?

I’ve tried adding the secondary email to the list of primary addresses and I’ve also tried removing the primary altogether. For the latter I was prompted to submit a new email address. Both options tell me that the email address isn’t available.