After spending rather a lot of time on this. (For my beloved younger sister)
And this. (For me)
My poor sewing machine was literally collecting dust! I do have a great project coming up, but in the meantime I got on with this quick fix.
Pull on stretch jeans with ludicrously low rise. ($5 from Kmart)
Unpick folded over casing.
Sew on new facing to top edge and understitch.
Fold in facing and stitch down.
Re-insert elastic, slightly shortened to allow for jeans sitting a little higher on torso.