The mid-length carbine is not just about adding a longer handguard; it's about changing the placement of the gas port (hole) in the barrel during the manufacturing process to get a different pressure curve, and moving the gas block or front sight frame to the new gas port location several inches down the barrel, which then requires a longer handguard. I know of no way to make a standard carbine-length gas system into a mid-length gas system without changing the barrel.
Changing the barrel requires special tools and fixtures (and know-how), and it's far easier (although much more expensive) to just buy a new upper receiver/barrel assembly already manufactured to mid-length specs, and snap it on your existing lower receiver assembly. You can then sell your old upper receiver/barrel group to partially offset the cost of the new upper half.
Or keep it, and eventually buy a new completed lower assembly to build a second AR...
...not that I know anyone who's ever done that.