Well yeah. Sifu can do that....at least if his the head guy. Now if us ordinary mortals were to do that, it's another thing.Ive seen my Sifu make changes on the spot, to a form.
My old sifu made changes to each of Yip Man's forms, even though he studied under Yip Man and was one of the first to possess the old film footage that we have now all seen on Youtube. I was present when he introduced certain modifications. They were not done because he wasn't taught the older version, but because he chose to make those changes.