Yes, I've done this alteration from time to time.
Not a job I enjoy.
I've never needed to take a huge amount out though and I don't ever remember needing to mess about with narrowing across the shoulders.
I hate it when there's that annoying little side split at the hem to put back with a separate little strip of binding to re-attach.
I've always simply stitched straight up the side seams, through the underarm and I try to run off to nothing at the sleeve hem.
Good idea to try on an older, least favourite one to start with if you're not sure about getting involved.
Sandra.
xxx