I've tried buying thread that looks like the special jeans hemming thread, but nothing works. Everything's too thin and doesn't quite look right.
Somewhere on the web, I read someone's tutorial for hemming jeans while keeping the factory hem and I'm really happy with how it works, so I will share my version here.
Since I already had a pair of jeans that I had hemmed to the right length, I just took the new jeans and matched them in the exact places (these are the same exact jeans, so it worked) and got to work. If they were a different style or brand, or if I hadn't made a pair to the right length, I would have tried them on, pinned them, tried them on, pinned them, tried them on, pinned them about 40 times.
You might have to do that too.
Then I like to cut them right where the factory hem is going to go, where I put the red arrow.
You're left with this.
Also, these particular jeans have one style of topstitching on the outside of the leg and a different one on the inside. If you're trying to keep this looking original, make sure you get the right one in the right place. Not that I'd know anything about putting the wrong part on the wrong side...
Usually, I have this at my machine to help me sew, but for this project, you actually want to take it away, unless you have a machine that's recessed into the table. It will still work, but I like to put the leg opening right over that part of the machine to keep everything from shifting.
One pair of jeans that I hemmed this way insisted on flipping up, so I've devised a way to sort of prevent that.
One word of caution. Since you are only securing that flap on each side, that leaves a lovely amount of fold to get your toes stuck in if you are trying to throw on a pair of jeans really quickly and you might take a tumble. Not that I'd know anything about that firsthand....
I haven't done this on any other kind of pants but if you wanted to do it on something like a pair of chinos, I would say you should only do it on darker colors. I think it might be more obvious on a pair of khakis.
I did this same technique recently when I hemmed the sleeves on a jacket for someone and it came out fine. The original cuff is still there and I attached it just like this and loved how it came out.