Apple ID is being showing as "already in use" after it was changed...

Here is my situation:

I've had a long standing, primary Apple ID that features an email I haven't used since I set it up. So when I found out I could change the email attached my Apple ID, I tried to do that. I found I had an Apple ID that already used that email, so I logged into THAT Apple ID and changed the email associated with it.

Well, I went to change my email in the longstanding one to the one that HAD been used in my other one, but despite being changed (and even confirmed) it's telling me it's already in use. Is this something that takes time to register? It's pretty frustrating.

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