A few years ago, Husband bought a pair of inexpensive jeans that ended up being a bit small. Instead of returning them, he gave them to me. At the time, they were a bit too small for me, too, so I put them away for The Future.
When I finally tried them on after losing weight, they...were too big.
I considered donating them, but the fact that they're relaxed fit and the waistline does sit at my waist means they're quite accidentally in fashion, so I decided to try altering them.
I consulted several websites that said all that needed to be done was pick out the stitching holding on the waist band (and any belt loops in the way), cut out some of the center back, then sew everything back together. There may have been better and more detailed instructions than that, but that's what I took and ran with.
I tried on the jeans and pinched out the excess in the center back and safety pinned that in place. I then (well...a few weeks later) used ore safety pins to emulate a line of stitching, then tried the jeans on again. Things felt like they where where they needed to be, so I used my index finger width (which is roughly 5/8") to draw a line with tailor chalk parallel to the pins, and that's where I cut off the excess fabric.
Due to the way I'm shaped, taking out the center back excess but
leaving the front intact still leaves the side seams right where they're
supposed to be.
I ended up opening the entire center back seam, and a bit of the inseam, which was easier than it should have been, because it was sewn entirely with safety stitches. All I had to do was cut a bit, find the right thread end, and, zoop, the stitch pulled apart.
I loaded a yellow 'gold' medium weight thread into the top of the machine and left the yellow green in the bobbin in place since it was the same weight. The color and stitching are far from perfect, but no-one's going to look very closely when I'm wearing these.
I also very quickly decided that I was absolutely not going to do a welt seam. I've never been good with welt seams, and I don't think my machine would have taken it well, either. When I sewed the inseam back together, I just top stitched through everything. My serger definitely wasn't happy with what I'd asked it to do. But, again, no-one is going to be looking.
I haven't worn jeans for a very very long time. This could get weird.