There's nothing tricky about dismounting a Lee Enfield barrel, it's pretty straight forward. However, to rechamber the barrel you will have to take 1 complete turn of thread off the end of the barrel and a corresponding amount off the shoulder to allow the Nocks form to be where it should be. All in all a nice looking rifle and quite worthy of a bit of effort to bring her up to what you want her to be. Sorry I can't help with the rear sight, I've not ever seen one that mounts on the left side of the action.

cheers
P