Do you still need them to stretch when getting them on?
If not I adjusted a top of my Mum's by going from one seam to the other picking up bits of the material with the needle. I used quite large stitches and at the end pulled the thread so the the material bunched to the size needed. It looks good as the material can still move so you can adjust it to sit comfortably.
Not sure if this would work for elastic waistband but there is no harm in trying.