As a matter of fact, I just replaced the wall return fitting on our pool without draining. I thought I'd lose a lot more water than I did and that I'd be soaked when I got done. As it turned out I only lost about 2 gallons of water.
First I took a couple of winterizing plugs and plugged the existing fitting and the new fitting. Then I cut the piping off the existing fitting and took the big nut off (I wound up having to cut it into and pry it off with a screwdriver). The water pressure held the existing fitting in place and it was only leaking drops. Then I got the new fitting in one hand with the rubber gasket on it and grabbed the existing fitting and pulled it out and immediately inserted the new fitting. Again the water pressure held the new fitting in, leaking only drops until I could get the big nut on it and tightened down. Then I reworked the piping and once the glue cured enough I removed the plug and replaced the eyeball.
All is well.

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Did any water get between the wall and the liner? Or did the pressure hold ot tight