Changing mouse sensitivity, while in the game, & ON THE FLY?
Tall order... you've got me!
APK
P.S.=> I know of a USB mouse tuner, which modifies USB mouse polling rates, & you use it in Windows!
Someone from this forums turned me onto it in fact, it works! I can give you the name IF you like, but what's the point? It doesn't FIT your definition requirements here!
(And, of course, you have your native Windows Control Panel Mouse item, which can increase PS/2 mice polling rates)...
HOWEVER/AGAIN: In the game, & "On-The-Fly"... whew, ask for it as a feature in your next fav game maker's game, or find a game that's "macro-able/scriptable" I would guess... apk