When a piece of clothing does not fit right or simply is not the style you currently need, altering a garment is an option. You do not have to throw out an item and buy a new one to get what you want. Switching out a zipper waist to an elastic back takes only a little time and basic sewing skills.
Things You'll Need
Elastic (1-2 inch wide depending on garment, length depends on waist size)
Use a seam ripper to remove the seam attaching the zipper to the fabric. Pull the threads out individually with the pointed end, or use the cutting edge of the seam ripper to cut the threads. Remove the zipper from the fabric when all the seams are gone.
Pin the free edges of the fabric together, matching the edges to ensure an even seam. Remove any pieces of thread still stuck to the fabric from the old seam.
Sew the fabric edges together, matching the width of the seam below.
Fit elastic band around you waist to measure how long it should be. Elastic should be snug on waist, not loose. Mark the length and add 3/4 to 1 inch for a seam allowance. Cut the elastic.
Insert elastic through the existing waistband. Pull all the way through and sew ends together, overlapping the elastic about 1/2 inch. Stretch waistband until elastic is completely inside waistband.
Top stitch the open section of waistband closed. Cut away threads to finish.
If you do not have enough free fabric to sew the fabric together once the zipper is removed, you can take out the seam of the waistband near the zipper area until you have enough free fabric to piece the ends together.
Going from a zippered waistband to an elastic waist band may not leave very much room for the fabric to stretch. You may need to let out seams or add in additional panels of fabric.