Old Apple ID showing up in iCloud

I need to erase my phone so I can get a replacement. In iCloud, Find my phone, when I go to erase it asks me for my password. Problem is, the apple id that is prepopulated and can't be changed is my old ID so of course it won't work. I tried using my current password and it said your account is locked so I went through the process to change the password but of course, I got the error "not a valid apple ID" which is correct becuase it is my old ID.

Any ideas?