There isn't much to removing the factory barrel.
You will need a heavy vise, aluminum barrel blocks and an ar-15 barrel extension wrench. You can buy the wrench from brownells(
http://www.brownells.com/aspx/NS/store/productdetail.aspx?p=12729&) or make your own with an old ar-15 bolt.
If you buy the wrench from brownells you will need to turn the outside diameter down so it will fit in the Ar-180 receiver.
1-strip the upper including pulling the front sight/gas block off and handguards.
2-make a mark with a fine punch on the front of the receiver and a matching one on the barrel for aligning upon reinstallation of the factory barrel should you choose to do so.
3-clamp the barrel tightly in the vise with the barrel blocks
4-slip the extension wrench into the receiver and engage the wrench to the locking lugs and turn the receiver off of the barrel(standard right hand threads).
--If you try to unscrew the barrel off of the receiver by clamping the receiver and turning the barrel you will just spin the barrel extension within the receiver and never get anywhere.
Regarding the new barrel. The threads are the same as on an Ar-15 but remember that the position of the gas hole is in a slightly different location so it isn't practical to try to fit a standard ar barrel unless you can get one without the gas hole drilled.
Quite a few calibers could be fit into the Ar-180 including 9mm, any of the .223 wildcats, .300 whisper(real fun!) and I am sure some of the PPC(?) cartidges.
Thats about all I can think of for now.
-Josh