I removed the casing stitches and elastic and sewed down the new elastic over where the old one was in the seam allowance. In retrospect, I might just remove a bit of the stitching at the top and bottom of the casing and thread new elastic in there and tack it down at the top and bottom of the casing.
The back was a little trickier, as there's more to undo stitch-wise, but if you can open the ends to make a tunnel, you can do the same and thread the elastic in and tack the ends.
As for the velcro, I took mine off and put snaps on, which made me like the diaper about 10x more.
