If this is a rifle that has already been broken in, then you are best suited staying with the old bolt that has mated with the existing barrel extension. If you did install a new bolt, then it would need to mate in with the old barrel extension, and would needed to be checked for headspace.

In regards to the new carrier, if your lower receiver has the old style colt sear block, then the block would need to be removed in order to use anything else then the open bottom C carrier. If you colt lower receiver does not have the colt sear block (raised unit), but has the new style block (non milled cross section just in front of the upper rear lug cavity), then the M-16 carrier will fit, but you may run into problems if the rifle is on the verge of correct cycling (read functioning gas system pressure on the low side).

Truth be told, unless you are having problems with excessive recoil due to the B/C-buffer moving too fast/ too strong to the back of the receiver extension, then adding the M-16 carrier may cause more problems then good.  If your reasoning is deactivate the safety system built into the colt system, which is the bolt not locking up if the disconnector fails (read prevents hammer follow threw), then you are taking a step in the wrong direction.  If you do have a disconnector failure and the hammer catches the firing pin collar, you can feel the irregular cycle, and know that you need to clear the rifle.  Without the feature in place, then the bolt will lock up, but the hammer has followed the carrier down and when it is time to pull the trigger, you will have a Fail to fire on, and left to clear the weapon at a less than ideal time.

The key/cam is only portion of the carrier that has to mate with the upper receiver, and the bolt will need to mate with the carrier gas section.  If you used the old key (already polished/mated), and the old bolt (at least polished but will still need o polish and mate to the carrier gas section), the parts should be good to go in a hundred rounds or so.  This will allow the bolt to polish/mate into the gas chamber section, and confirm that the key is not leaking (remember to torque the allen head screws to 35 in lbs and stake the key metal into the allen head side spines.

On the other hand, if you are going to go with a complete new built B/C, run the rifle for at least a few hundred rounds to allow the new compounds to make the old ones (read completely breaking the rifle in like you did when it was new).

In regards to Colt using the heaver carriers (?), as long as the gas port is correctly sized for the more mass/heavier carrier, everything should be fine.  If you have read threw the trouble shooting threads over the last few days, you will see a theme that is growing with guys running the Colt M4 SOCOM 14.5 1/7 Barrel ported at .060’ish having problems when running ammo loaded with less than Nato powdered loads (read barrel is designed to run full Nato pressure loads suppressed and non suppressed).

Again, if the carrier is being installed to compensate for excessive recoil (buffer striking the back of the carrier with too much force), then going to a heaver carrier will help to alleviate the problem.  On the other hand, if the rifle is cycling fine with the lighter carrier, then you may be forced to enlarge the port up to the next size, and may be a option that Colt is taking if they are going to run with the heaver carriers and still have the rifles perform with lighter loaded ammo.