I don't think you need to save so much water.
We do massive water changes when the fish are stressed. A massive water change shouldn't be that stressful so long as the temp and chemistry is what they're used to. There isn't a significant amount of bacteria to save in the water column.
In both cases, you'll have to move enough water to move the fish in. If that works out to a quarter of the total volume I'd call it a 75% water change and spare my back.
We do massive water changes when the fish are stressed. A massive water change shouldn't be that stressful so long as the temp and chemistry is what they're used to. There isn't a significant amount of bacteria to save in the water column.
In both cases, you'll have to move enough water to move the fish in. If that works out to a quarter of the total volume I'd call it a 75% water change and spare my back.