I'm not a shrimp "E-guru", but I had a colony go from a dozen or so to many thousands spread over a number of tanks, and removed/sold/traded hundreds as well as using many more as feeders. They thrived despite the Neanderthal-style care they received, which was the same as all my fish. It involved draining down all the tanks to within an inch or two of the bottom, with fish often laying on their sides, and then filling up with brand-new, untreated well water.Sometimes there’s not a choice you need to take some stuff out to net a problem fish. The plants will be fine lowering the water. To answer your question draining the water out and keeping it to refill the tank back up is totally fine. In fact with invertebrates your better off not doing a major water change or you run the risk of stressing out the shrimp and causing them to all molt also any berried shrimp will loose their clutches. It is definitely in no way equivalent to reusing motor oil.
Delicate invertebrates are always thought to fare poorly with large water changes. I'm sure that might be true if the changes are few and far between; the water in any tank degrades over time, and then suddenly changing most of it exposes the critters to big abrupt shifts in parameters. But frequent massive changes don't do that; weekly, or even more frequent, changes mean that the water is replaced before it has significantly altered in chemistry, so the animals are living in stable parameters rather than wildly swinging ones.
Don't change the water when it gets old; change it before it has a chance to do so.
I'll stand by my viewpoint that if you are going to the trouble of removing 90% of your water...as long as you do it regularly and often...the best use of that time and effort is to change it at the same time. I might think differently if my water was treated and reverse-osmosed and then re-mineralized and artificially buffered and pH shifted and who-knows-what-else...but we don't it that way in the caves.
