Upgrading parts like that is not entirely unheard of... They did that with the eeepcs . As long as dimensions are the same as well as pin compatibility, then i cant see why it wouldnt work...
I've only had to change out the digitizer myself on the 3G and it wasnt as bad as it looked . Just have to take your time . I would like to change out the battery in my 3g , but haven't looked into it yet . After reading some comments in here I am assuming it is harder to do then on the new 3GS version .