The only alterations I've done was with jerseys with elastic around the sleeves or hips. I completely removed the stitching, got rid of the elastic and then re-hemmed.

I was offered a "unisex" club jersey last year in return for leading a certain number of rides, but it was ridiculously narrow in the hips. Even a size large, which was comically to big everywhere else, was too tight around the hips. I ended up saying thanks but no thanks. This year they were able to give me one that isn't quite so bad. I still might remove the elastic, I haven't decided yet.