I have a favourite pair of denim capri pants that I practically live in. However, the zipper has always crept downwards, especially when I need to bend forward. The other day, I was watching this 'Sewing with Nancy' episode on my local PBS station where she showed an incredibly easy way to replace the zipper without having to undo any seams. The relevant part starts at 13:38 on the video.
I got so inspired, I went right out and bought a replacement zipper. Today, I sat down and did the replacement.
This technique is unorthodox in that you simply cut the faulty zipper out as close to the stitching lines as possible. Then let any remaining zipper fabric unravel and trim off any errant threads. The decorative topstitching is unaffected, although it no longer holds a zipper in. I also cut the two bar tacks holding the zipper flat.
This freed up the two parts of the bottom of the zipper placket.
Folding down the top of the zipper tab out of the way, I pinned it into place (Nancy uses two-sided tape instead of pins in the video but I didn't have any), and using denim-coloured thread sewed the two sides of the zipper into place.
as the flap completely covers the zipper tape.
The new zipper doesn't creep! Hurray!