Quoted:
You can change the barrel, the chamber is part of the barrel.  

Unless you're planning to register it as an SBR though, you will have to have a 16" minimum barrel.  You can use the 14.5 but you'll have to get a muzzle device long enough to get to the 16" and have it pinned and welded in place to be legal.

Quoted:

Understood, thanks.  Can just about any muzzle device be pinned/welded?  I'm looking at linear compensators.

Is it a big deal to go from the carbine length to mid-length gas system, or is it as simple as buying the shorter gas tube?

Quoted:
I've never done a pin/weld myself so I can't say for sure.  There are probably some muzzle devices that are more user friendly than others to work on than others.

No big deal going from carbine to mid-length, seems to be a matter of preference with carbine supposedly being more reliable...supposedly.  Mid-length supposed to soften the recoil impulse.  If you're moving TO a middy, then the tube will be longer than what you have now.  You would have to knock out the roll pin in the gas block to remove/replace the tube so you might consider just getting a new gas block as well to avoid any 'oops' there.

Since you're new to ARs, and I'm not sure if you have the necessary tools to prevent nightmares, you might consider just buying a new assembled upper in the configuration you want and spend 20 seconds putting it on your lower as opposed to having your first mod be a barrel change.  Much easier to go back to square 1 in case you run into any headaches with the work or how it ends up shooting.

Something like this is close to what you're after.  Get a new bolt carrier and charging handle and you have a complete second upper.  I didn't see where this flash hider is pinned so you'd still be looking to get that work done immediately to stay legal.

Quoted:
If you do get a 14.5 with the intent to pin and weld a device, make sure the device extends the barrel past 16" when attached. Some are too short to accomplish this.