Yesterday I shortened the sleeves on a coat that had a vent and non-working buttons, i.e. The buttonholes were not cut. This is a complex process, and I thought I would explain how I do it.
I don't touch shortening (tailored) jacket sleeves from the top because there is usually a lot of 'engineering' there that is difficult to unpick and reinstate. Sometimes there is a sleevehead, and sometimes the body and body lining are attached to the sleeve and bound before the sleeve lining is handstitched in place. Anyway, the whole thing is prone to error and looking nasty. Providing the amount to be shortened is not too much, shortening from the cuff end is much easier and looks fine.
Before I start, I mark the new hem length with tacking (basting) stitches, being careful not to stitch through the lining.
When you shorten jacket sleeves this way, you will need to remove all the buttons and the fake buttonholes and the stitching for the vent edge to be able to turn the new cuff up. Before taking the buttons off, measure the position of the first button from the seam and the cuff, and also the distance between the buttons, so you can resew them in the equivalent position when reattach them. Also measure the amount the cuff is being shortened so you know how much fabric to cut off the sleeve (and the lining).
Go in through the underarm seams even though it feels like you could just separate the lining from the sleeve at the cuff. Usually one of these seams is stitched last after the jacket was bagged, but open both up because it is less stress on the jacket.
It is usually easier to undo the buttonhole stitching from the inside as they are sewn with a special stitch that can be unchained from the back.
Add new interfacing to provide support to the fabric at the marked length. Don't forget to interface a little further up under where the buttons will go.
Since the sleeve tapers towards the cuff, you will need to renew the non-vent seam in the new hem to angle outwards, to allow the hem fabric to lie flat. Then you can work out where to resew the vent stitching. Sometimes there is a mitre seam on the overlap side, so use pressing and pins to work out where to put the new one. This sometimes needs to be at a different angle to the original.
I usually don't bother with new fake buttonholes as they really aren't noticeable, but if you want to make new ones, use the measurements for button placement that you made at the start. Just sew them onto one fabric layer as they will be fakes - if you are shortening, the extra layer needed for working buttonholes just won't be there.
Trim the lining fabric by the same amount you are shortening the sleeve. I usually find I need to resew the lining seam with narrower seam allowances at the cuff end to match the width of the hem edge.
Sew the lining to the cuff as you do when bagging a jacket, by pinning the two edges together as they will be worn, then passing them through the open lining seam. Catch the hem in place at the seams (or hemstitch all round).
Resew the buttons in place at the measured positions but not through the lining.
Finally topstitch the lining seams closed.