When I was hiking The Desert Trail the zipper blew out on my Montbell Alpine Light down jacket. I needed that jacket, the desert got cold at night! I stopped in some kind of sewing store in Lovelock, Nevada, right on the trail, and asked if they had a zipper the right length. They did.
When the store owner found out I was going to sew it on by hand she offered to let me use her machine. I took a look at the seam and it looked like a complicated process to pick out the zillions of tiny stitches and then deal with a possible snowstorm of down.
Instead, I used your idea. I cut the teeth off the one side as short as I could. It looks like I singed the cut edge to keep it from fraying. The other zipper side was very short so I just left it in place. I sewed both zipper halves on top of the old zipper. I think it looks like new. The lady couldn’t believe a man had done the job so fast and neatly. She went and got her friend to show her.
I’ve used a similar method on other jackets where, for one reason or another, taking the seam apart didn’t make sense.
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