There is a way of hemming jeans that leaves the original hem intact, and the new seam is practically invisible. You have to have a good, heavy-duty machine to do it though, because it has to sew through the side seams. This is how I've been hemming my family's jeans for the last few years.
A written tute I found:
http://www.daciaray.com/?p=38
And here's a video tute, might be easier to follow:
https://www.youtube.com/watch?v=giOkJUcc2wg
I use my serger.