Using a 40 flap Cessna as an example, though knowing that different type's flaps do have differing effects, the first 20 or so flap of the Cessna does more to increase lift, than to increase drag. From 20 to 40 does more to increase drag than lift. But, even to 40 flap, there is still greater lift than with zero flap, just accompanied by a whole lot of drag. If you were to be approaching with 40 flap way back ('not sure why), and needed to stretch the glide, than yes, you could retract to 20 flap, as one would for a go around. In doing so, you will reduce drag (good), though you will loose some lift (not so good), but also, you will increase the speed at which the wing would like to stall. So, if you were gliding slowly with 40 flap, and retracted to 20 or less, your stall speed would increase, and place you closer to the stall. In that case, you'd have to lower the nose to build up some speed, and increase your margin to stall, and in doing so, you'd probably loose enough altitude to waste whatever you would have recovered by the reduced drag. The pitching moment of the wing changes, and you'll spend some time and energy adjusting and retrimming.
Thus, for any but a very short landing power on, approach, I will not extend more than 20 flap in a Cessna, until a safe landing somewhere is assured. But, I do always plan to land with full flap, so I set up my approaches with that in mind. When landing a Cessna floatplane into a small piece of lake (often, you want to keep it close to shore, to avoid the big water), I'll usually have full flap applied before reaching the last row of trees coming over the water, I'll be steep and slow, so the plane drops over the trees well. Though I could glide on from there, a little burst of power makes the landing less extreme, and safer. Otherwise, I like approaches, where a gliding landing at least in decent ground up to the end of the runway would likely work. So I won't extend full flaps till I have it made. When I used to train new pilots in the ATC810 twin simulator (Navajo sort of thing, primitive by today's standards, and no visual whatever) full flaps were a deliberate trap, by design. Once full flap was extended, if an engine was failed, a successful single engined overshoot was nearly impossible - just to drill into pilots that full flap is great, once you are very confident of a landing as intended.
I'm sure that someone wiser than I can put numbers to this (Genghis?), I just know the sensations from doing it a few times! If in doubt, refer to the POH. It's going to give you the manufacturer's preferred technique. You'll never be wrong flying it that way!