Transferring old iCloud email address to new account?

I was in the process of creating an iCloud email address for myself (in order to set up a custom email domain), and I was surprised to see that the iCloud address I wanted to use (an unusual one) was already taken. For the sake of this post, I'll call the email address XYZ@icloud.com.

I tried to log into XYZ@icloud.com using a password that I frequently used 20 years ago (repeating passwords is bad, I know, but it was 20 years ago). I found that XYZ@icloud.com was locked but was, indeed, linked to an Apple ID using that address about 20 years ago. I was eventually able to get access to my 20-year-old Apple ID account, and update it so that is linked to a current phone # and email address of mine.

Now that I've done this, I'd really like to transfer the iCloud email address XYZ@icloud.com that is associated with my 20 year old Apple ID to my current Apple ID. But, I don't see any way to transfer the email address. In fact, I'm not able to actually access XYZ@icloud.com. When I log into the Apple ID that is associated with it, I get a message saying that it is a "web only" access account. I added the new account to my family plan, but it still shows as web only. So, the address XYZ@icloud.com exists and is associated with my 20 year old Apple ID. But, I have no way of transferring it. (I assume that if I were to associate my 20 year old Apple ID with an Apple Device then the account would no longer be web-only, but I don't want to unlink my current account on any of my devices.)

Does anyone know if there is a way to transfer my old iCloud email address to my current Apple ID account?