Why would it run worse? U can still play PC games from 1992 and they run just fine, look terrible but still work.
And if they did have to change it, then make sure its a good change and ready from the start and as close as it can possibly be to the game considered perfect gameplay wise. Lets be realistic, nobody asked for theese graphics or this retarded spraying or shittier movement, people asked for better anti cheat, better MM ranks/leaderboards and rewards, and 128 tick servers. But they gave us none of those and decided to do what Valve always does, put some bs on us and then see what happens.