I don't believe that the 6502 had such a thing as a positive/negative bit that would have this kind of effect. On that hardware, if I was going to do this, I'd

I don't believe that the 6502 had such a thing as a positive/negative bit that would have this kind of effect. On that hardware, if I was going to do this, I'd just reverse the order of the entries in the scanline lookup table. On the Apple II the address of the first pixel of each scanline did not follow immediately after the address of the last pixel of the prior scanline. So every program that did any graphics work used a lookup table to get the address of a particular scanline. As long as you were consistent in your use of this table, that would be the only necessary change to flip the game upside down.