You can replace the barrel extension... it is going to be much,much, easier to just buy the right barrel. You need the barrel extension "socket" and barrel jaws to remove the extension... then when you put the new one on your going to have to time it just right with your front sight base so you have no cant. Then I dont know what kind of headspace issues you might run into.
Ive never replaced an extension, but it looks to be a super pain. You could probably get somebody like adcofirearms to put an m-4 extension on a RRA barrel... but you will probably spend more doing that than just getting a barrel with it already on there.
Then.... there is the dremel tool........but if you botch that job up... your going to need a new extension. My advice... get the right barrel for the right job.
Later,
Ren