I'd caution that this only works if the old hem and the new hem are the same width. Do not try with bootcuts! As others have mentioned, curling/flipping is an issue too especially if the hems are not exactly the same size. You might have to press your hems every time you wear them.
I hem my own jeans these days (honestly the tailor is $$$$ and didn't do a great job anyway). I don't have a sewing machine so I do it by hand. I just fold the excess length up and sew a straight backstitch across the bottom. It looks fine. I generally prefer to keep the extra fabric (without cutting it off) so that I can let it out if the jeans shrink or something.
