Making low-waist bottoms into high-waist most efficiently?

Hi. You don’t need to edit the mesh but you’ll have to edit all maps, they have to match everywhere.
You may also have to edit the sort layer (compare the sort layer of the item you want to edit with the one of the maxis high-waist pants).

This is how to do it in photoshop or photopea (free online program that is almost identical to photoshop)
- export a diffuse texture in png, open;
- select about this much of the texture(until it reaches dark shadow in the crotch area), Edit - Copy, Edit - Paste - Paste in place;
- Edit - Free transform, move the selection up, deselect;
- choose Eraser, switch to the soft brush, remove the obvious differences. If you will have to expose unwanted fine details, that's ok, just make sure there is no difference in, like, shades;
- merge two layers;
- choose Clone stamp tool, sample the area close to the unwanted detail with Alt, and remove it. Do not worry about mismatching colors for now, it will be easier to even them out when they are close to each other;
- select the area with those mismatched colors, RMB-click, choose Fill. Choose content Aware, hit Ok. If the result is not good, try selecting smaller area, further away from pockets or things like that;
- if you will still be getting bad results, try Healing brush or Patch tool. You can use healing brush to remove repetitive wrinkles in the groin area.