Thank you so much! Here's what I did/learned. I went to the iforgot website and put in the troublesome old mac.com appleID, and went through the recover my password option. Luckily I had long ago updated the contact email associated with it to be a real one that works (@mac.com ones went away years ago), so I got the reset link. Clicked it, and tried to just use the same password I use on my other apple ID. It wouldn't let me, then the link expired and I had to try again.
Take 2: I put in a fresh password on reset and was asked to "update/verify my email address", and was presented with a blank that already had my mac.com apple ID in it. However, because it was asking me for an email address and NOT my ID, I put in my main email address (the one on file where password resets are sent already).
Unfortunately, it wouldn't allow that email in the blank. Don't know why, but it was either because that email was already in the system as my contact info (this seems unlikely) OR it's because I'm already using that email address as my other apple ID (which I mentioned I had to make years ago for some reason). So, I put in a different email address and success!
EXCEPT. That new email address has now BECOME my apple ID! All my devices that were logged in with my mac.com ID show up there, and the old ID no longer allows me to log in. It's like Apple finally took the last step to eradicate @mac.com email based IDs. I'm fine with this except for the fact there was no indication that I was literally changing my apple ID when asked for an email address.
All other apple services have updated automatically and now my old ID no longer works anywhere (icloud, iTunes, app store, etc). Thankfully, the new ID does access all that stuff so I'm good. Weird, not ideal, but I'm moving on, hahaha
