A question from someone who doesen't know a thing about modding M2: TW

Greetings, fellow Total War fans! I've never modded Medieval 2 before, to be honest. Still, there is a mod I've always wanted to do for it. I want to change the current missile model files for all crossbow using units in the game , including those in the expansion, and replace them with the crossbow-bolt models from RT: BI. I'm so disturbed by the fact that the Creative Assembly made the crossbowmen in M2: TW fire arrows, instead of bolts. I'd also like to know if it is possible to give the Medieval 2 crossbows the same velocity as the ones in Barbarian Invasion (because it looks more realistic that way). If someone could link a tutorial, or help me in some other way, I'd be forever greatful!