Changing barrels

Headspace is set by the barrel. You may need to change your buffer along with the other items you listed.

 

As mountain man said, the buffer , spring and tube will have to be changed to rifle length.

 

If you have the standard carbine stock, buffer, etc. you probably can use it with the rifle length barrel.

As said before, headspace is set by the barrel and barrel extension so you shouldn't have to worry about it. Have you thought about just getting a 20" URG and selling your 16" URG?

 

If you have a carbine stock set up, the 20" barrel will just look wrong. The balance will be off. IMHO, fixed stock - 20 inch. Collapsible stock 16" and under.

 

The stock thing is subjective. Some people like to keep the collapsible stock

 

Handguards and gas tube are the the only real mechanical parts you need to change. The stock, buffer, buffer tube and spring from a carbine or a rifle should function fine. Some folks don't like the look of a carbine stock on a rifle. Some folks like to keep the option of adjustment for smaller statured shooters and large shooters to be able to use the same rifle or to be able to adjust for warm and cold weather clothing. The Canadian military issues a rifle length upper with collapsible stock for those reasons.