For years I have used elastic to tighten up all kinds of waistbands. I've added elastic to jeans, dresspants and even pants that already have an elastic waistband. Recently I "saved" a beautiful skirt that I had just finished making for my GD. After all my work making the skirt the elastic waistband was too big. It was a wide pretty metallic elastic. So to make the waistband fit my GD I sewed narrower elastic to the inside of the wide elastic. I didn't have to rip out a single stitch and it was such an easy fix too. It was after that when I started thinking about posting this as a tip especially for new sewers so they will know fixing and repairing can sometimes be quick and easy. You don't always have to do it the way a book says.
If ever you need to remove the elastic it comes out easily because you've used a long stitch length to attach it. Just rip out the stitches on the short ends of the elastic, pull on the bobbin thread on each long side of the elastic--they fly right out and you're done!
Now, if a waistband had easy to get at elastic inside a casing, well then I would rip open a small section of the casing to get at the elastic and replace it or shorten it. But for all other waistbands I just add elastic to the inside of the waistband like shown on the photos.