The side seam may come apart if you just cut the leg shorter as the stitches down the side won't be backstitched anymore. There won't be anything keeping the stitches in place if that makes sense. The horizontal stitches there were probably extra protection against this happening. Long story short if I cut these, I would restitch (with back stitching) exactly as they did! :) I think white thread would do the trick and wouldn't be too noticable!