Yes, but why?
Do you just want the shorter length handguards of a carbine? The mid-length gas system will give you smoother recoil impulse over a carbine, less wear/tear on your system, longer sight radius - all good things. If you are just trying to change out an A2 front sight, no problem. You can remove and replace it with quite a few different front gas blocks, with or without rails, some even with flip down front sights.
Check out these links for a few different options:
AR-15, M16, M4, Front Sight Bases
PK Firearms | Online Store
Rainier Arms, LLC™ | Manufacturer
YHM, Yankee Hill Machine Co., Inc. – Makers of High Quality Firearms, Accessories & Sound Suppressors | Sights
The longer forearm of the mid-length also allows you to extend your grip out farther on the forearm, which leads to more accurate target transition/acquistion on the move. Of course that is just my .02
I guess I'm just a little confused as to what you are trying to achieve going this way. But hopefully the above links help you out. It wouldn't be too hard to change out the front sight and go to a railed gas block or one that has a front flip down sight built in. But if you leave the gas block where it's at, then change to carbine length hand guards, I think it's going to look a bit odd, to say the least. If you are trying to totally convert it to carbine length, you'd have to weld/seal the gas hole, relocate and re-drill a new hole, in the correct location for carbine length gas impulse. It would be easier to just buy a new carbine length upper or barrel assembly, most likely cheaper than paying shop prices to do the modification. Again, just my .02.