Nothing wrong with a 16" , there are a couple of Threads on here of 16" Barrels & their accuracy against other lengths . I have a 16" HB Barrel & my testing out to 200 yards ( as far as I have compared the two ) compared to my 20" HB is ,there is no real difference in grouping between the two Barrels.
If you do change it out , as said here , keeping with a Armalite configuration Barrel , you should get a new bolt with the new barrel anyway , so going to a DPMS config. Barrel , you would still need a new bolt for the new Barrel , so no difference in what components you would need .
Not a good idea to swap Bolts that are already seated to a particular Barrel . It would probably not be a problem , but what if , for some reason the used Bolt would not head space correctly on the new Barrel , why take the chance . New Barrel , new Bolt & still check Head Space, just to make sure , be it where you buy the Barrel or check it your self .
What uses do you plan for this rifle ? Hunting , Target , long range or a little of both ?
Putting accessories like a Free Float hand guard , Muzzle Brake/comp & good Fire Control Group , are some good ways to make a better shooting rifle .