I got some strange looks when I did it, but I have taken cycling jerseys to regular alterations tailors to be taken in. Most recently I used the same one I regularly use for my business clothing, the prior one was just a regular alterations tailor / dry cleaner. I also pinned my jerseys to get the fit where I wanted it to be in the riding position before taking them in.
I have turned several specialty club fit jerseys into more of an aero fit, which I like. Taking in a jersey should be pretty easy with the seams on the sides. I even had a skinsuit shortened vertically. They unattached the bottom and top halves, removed an inch from the top, and joined it back together.