No worries. I do have a great cobbler, and I considered going to him with the original. I likely will, because I have a softer, fattier shoulder pad that I'd like to put on the strap while one of the ends is loose. My first thought was to get Filson to cut one for me, but I quickly remembered, thanks to posts like yours, that my cobbler could do a fine job of it.
Yes, it's very easy. I just cut the main strap to my desired length and used a leather puncher to re-punch a hole. I found some brass chicago screws on ebay to match it. It was super easy and can't tell that it was adjusted.