Changes - graphics:
Copyright screen changed to a cute little title.
Graphics for ship/rocks/UFOs changed to approximate arcade vector graphics (not just hollowed out)
All colors changed to black and white (except player 2, who is light gray)
Shows reserve ship sprites instead of numbers. (from Sadistroids)
Changes - gameplay:
Rock horizontal speeds tweaked so they don't 'fly in formation' quite as much. game is somewhat tougher as a result, and a lot tougher in 'fast rocks' mode (though not as hard as Sadistroids)
Player ship has greater thrust (also from Sadistroids)
UFOs are no longer an option, and appear regardless of how the difficulty switches are set.
Special thanks go to Thomas Jentzsch, for the Asteroids disassembly and info, and for creating the Sadistroids hack, which I used as my baseline.
(been a while since I hacked anything, and this is my first hack greater than 4K)