Porting animations from CoD5/CoD4 to CoD2

[Tally]

Do I need to change something on the anims (weight-paint/rerig/change joints/rename etc.)?
Or is it simply a drag'n'drop and export of the anims from other CoD's to CoD2?

I'd suggest you watch Tom's video tutorial on it:

http://tom-bmx.com/downloads/?action=tuts&id=12

It's quite involved. You have to play around with a lot of settings.