Simply purchase the upper of your choice, push out the two take down pins on your rifle, remove Colt upper. Set new upper into place and push the pins back in. That should be all it takes unless you are going to use the same charging handle and bolt assembly in both rifles. They will swap out also but I am of the opinion that the bolt wears to fit and it is better to keep a bolt with it's barrel. I have swapped bolts with no problems.
Not trying to make this a joke. There can be some differences in older Colt trigger and hammer pins (think they are all mill. spec now) and of course the old large hole and small hole(standard size now) lowers but if all you want is a new upper, there should be no problems. If you were trying to change some of the lower parts there could be more information needed.
I personally purchase the new upper with charger handle, bolt, and carrier assembly. That way you only need the lower assembly when you build your next rifle......always happens. Should say that any brand should fit and function fine. All parts manufactuerd today hold pretty good tolerances.