I just did this very thing last week, and to clear up a couple things:
1) The Existing .15 motor mount will work just fine, so don't worry about changing it.
2) Buy the conversion kit, I bought mine for $35 from a hobby shop and it came with everything I needed.
3) Drilling of the old style TMAXX chassis is required, to mount the throttle linkage (2.5 is a slide carb, .15 is rotary) bell crank. (The tmaxx conversion kit (I don't know if there are aftermarket companies that make them) from traxxas has a template that you follow to drill, so it's not too difficult. However, a lot of dissasembly is required to drill the 3 holes you need in order to get them in precisely.
4) Really simple: reversing of the throttle servo is required.
5) when installing the 2.5, be sure to set your gear lash for the spur.
I think that's everything. The 2.5 sits a little farther back on the chassis, so adjustment is required, but that should be it. Hope that helps.