I don't think the change is as apparent as in tailwaters. Sometimes those fish really want one fly and one fly only. Example, I was fishing the S. Holston with a buddy of mine about two weeks ago at webb bridge. Dreary, cloudy day, with sprinkles coming and going. We caught a decent number of fish in about three hours on ptails, a sulphur nymph, red san juan, and zebra midges. We caught probably twenty trout between us (three of us) which is definately a good number of fish, but the fish were spread out and unpredictable. I had fished this section many times and knew where the best hole was and wanted to give my friend a good shot a big bow while I moved upstream to figure out which flies were working best. The hole I put him in which is usually packed with fish only produced one fish for him in about two hours. About forty-five minutes before we left, I re-rigged with an indicator high on the leader, 5x tippet to a brown san juan with a split shot above it, and 6x to a tan scud beneath it. My next three cast produced a fish. Then I missed one, then I caught another on the next cast. I rigged my buddy up with the same set up and he caught three bows from 14-16" in about twenty minutes with a few smaller ones thrown in. They were keyed in on both the worm and the scud. I've had the same scenerio happen a few times in the middle of the winter with a bwo nymph, where the fish couldn't watch it go by without eating it.
Another example of why I believe small stream trout are less picky comes from three experiences I've had in the last couple weeks on small streams. In the past two days I fished a creek that runs through campus here at school. The section I always fish is about six hundred yards long and I really pick the stream apart as I move up. I never saw any hatches, but I fished a para adams, yellow stimi, and some other flashy bushy dry fly which I don't even know its name (something odd, not a normal fly). I also fished a ptail, copper john, and royal coachmen wet fly. I never saw a lull in action on any fly. These fish are oppurtunistic as heck and take advantage of whatever they can eat given it looks natural. I also fished a stream about ten miles from campus with the same flies and had the same results.
Now I know this probably isn't the case all the time, and that small stream trout get picky at times too, but I don't believe it is as important as people make it seem. In small streams, its all about knowing where fish are sitting and not spooking them out of there before you get a fly to them. As long as you're getting good drifts and not scaring fish, you can almost call your shots on small streams, atleast to the point of getting strikes, I usually don't get a hookset on these small fish. I consider a fifty percent hookup day on small streams a good day.
This is a lot longer of a post than I originally intended on typing up, but just my two cents worth. I like the discussion.
"I've got to stop wishin, I've got to go fishin"