well, technically speaking all of the engines were "modified" versions of the older ones. it's not like they completely re-wrote the engine for each game, they just kept rolling in newer and newer code, replaced the old code to fit the needs of the game design, etc etc, just like updates to computer software and programs

I never understood what makes ODST so much different to warrant the "modified" engine label aside from it having less code changes rolled in than any of the other games