If I ran the universe my entire wardrobe would be 1/4 zip fleeces and outer coats. But rather than have the top 1/4 or 1/3 be a "zipper" they would be ties. Zippers are fragile.
Bill of Material
I used 24", #8, plastic parka zippers. I hand sewed them in with black, #69 nylon thread. #69 nylon thread has a typical breaking strength of 11 pounds.
I left the original zipper in place and stitched to the zipper tape of the old zipper.
It went fastest when I kept the maximum thread length to about 18" and I waxed the thread. The waxing not only made it easier to pull but reduced tangling. Tangling was a major PITA because I double thread and the thread kept looping and catching on the teeth.
They are pretty ugly when you look close. Purely by accident, the two coats were both black, as were the replacement zippers and the thread that I used. So you have to look closely to see the damages. I am not a perfect tailor but I can do functional repairs.
Both coats are substantially warmer now that I can zip them closed.