Yes, I have most of these... Centipede and QBert can be modded to play the other game (just a jumper on the board). Rampage is only Rampage as far as we can tell. Joust can be jumpered to play Stargate (although the loading screen says Defender).
Oregon Trail is made by the same company, with the same boards. I believe these are all 'famicom on a chip' games with different EEPROMs. The EEPROMs have been dumped, but I don't think anyone has tried to load other games on them...
Oregon Trail is exclusive to Target. The full RGB LCD versions of the other games are exclusive to Wal-Mart. Bought anywhere else (like Centipede) and you get the Tiger-game style LCD...
