Bannofi, it sometimes is possible to do it from the top of the sleeve, but then there are the issues of the fact that the sleeve gets narrower as you go down and that creates a lot of problems getting enough width to re-cut the sleeve head further down the sleeve.
I have actually done the method where you cut across the sleeve above the zip before, on my clients clothes. It is the method that involves the least work IMO, as you don't have to remove the zip and re-set it.
It will be expensive though, as Angie said