I went round and round with Oracle Support creating spin-off TARs and transferring it from one group to another. I finally found an article that said one of the 11.5.x versions was really executing fnd_web_sec.validate_login() under the covers.
Might as well give that a try, SINCE I'M STILL WAITING.
So I ran the function on my GUEST user and sure enough, it came back with a "Y" which indicated the GUEST user and password were correct.
I then tried the same for my SYSADMIN user and the function returned "N".
Hmm, that's interesting, let me try it again.
Sure enough, my old SYSADMIN password didn't work for some reason. I posted the results in the TAR and the analyst suggested I change my SYSADMIN password with FNDCPASS.
I changed it, and was in!
If this little tidbit of an upgrade is published somewhere, it sure wasn't obvious to me.
Of course, now I have to figure out why anything using the forms servlet won't come up. And how to use WebADI. And why my printers aren't defined.
But at least I'm in!