My Password No Longer Works

Hey guys, this may sound nuts but I cannot log into my machine after restoring it. Here's the long and short of it: My iMac had a bum HD so Apple replaced it. I was on 10.7.2 and completely my last time machine backup minutes before I unplugged it and took it to the Apple store.

I get the machine back and restore from a time machine backup, which took 6 hours. Once it's done I am greeted not by the 10.7 login screen (grey crosshatch) but by 10.6's purple cosmic background. Now the big problem - my password does not work for either account I have on the machine and I know exactly what it is and how to type it.

I am thinking there is a conflict in restoring a 10.7 time machine backup to a 10.6 installation. Does this sound like it could be the culprit? Not sure why the geniuses rolled my machine back to 10.6, but it's causing some major headaches.

I figured a restore would restore my system to what the backup had, period. Should I do a fresh 10.7 install from the disc and restore again? Would that likely fix it? I am weary as the restores take 6 hours a pop.

Would love to hear your thoughts.

Thanks.