I have done exactly what you describe on two rifles. Contrary to what Tac40 mentions, the headspace was not actually affected after reinstalling the original extention, but what was affected was the "clocking" of the extention lugs to the gas port. Tac40 is 100% correct if a new or replacement extension is used, then headspace and gasport alignment will be a major issue. In my case, the change was slight but noticeable when rezeroing the rifles after all the work was completed.
Should you decide to tackle this job, you will need a set of barrel viseblocks to hold the barrel when loosening and retorquing the extension. The proper torque is 150 ft/lbs.
I, too, encourage you to just buy a new preban barrel and brake to do your floating HG conversion. Makes the job a whole lot easier and perhaps less expensive in the longrun.
Opps. Just reread your post. With a 14.5" barrel you are going to have a real problem. After sliding on your new hand guard and or locking ring you have little to no barrel ahead of the sight post to grab with the vise blocks. You may have to clamp around the existing compensator to retorque your extension rather than the barrel itself.