I had an experience somewhat similar to what you asked about a few years ago.
During the flight from New York to Zurich, Dec 2011, I was seated in the Economy class, left row window seat, where the leg space is close to nonexistent. Being 7' tall (212 cm) I was in for 8 and some hours of flying in a very unpleasant knees-to-my-chest position and didn't look forward to it.
Half an hour after the plane got off I waited for the personnel to exit the cabin going on their various duties and moved to the first row of seats in the center (the one usually charged extra due to the extra leg space it provides). After the steward walked past me two or three times within the next 10 minutes, he rightfully came to the conclusion that I was not supposed to be in that seat and approached me asking (quietly and very tactfully) if the seat I am sitting in is mine.
I explained to the steward as best as I could that I was having a really bad time in the seat that was originally assigned to me, and that due to the entire row (along with the whole center block of seats) being mostly empty I sincerely hope I would not be causing any discomfort or unpleasantness to the few passengers sitting relatively close to me.
I guess being sincere and polite helped, for after explaining me that he was required to charge an additional cost for the premium seats because a) that was his job, and b) if he didn't then everyone would relocate to the better seats, he asked me to give him a "credit card" (which happened to be my library membership card) from which he theatrically "deducted the additional cost" of my new seat, handed it back to me, wished me a pleasant flight and continued on his regular duties. I finished the flight in my new and much more comfortable seat.
TL, DR: As long as you are well mannered and have a sensible explanation for your request, there is a good chance that your needs will be met without much hassle, as long as you pay good attention that, while doing so, you don't put the person helping you in an uncomfortable situation. They are just doing their job, and it's in their best interest that you have a nice flight and have the urge to fly with them again. If they can help you, while not exposing themselves to scrutiny, in my experience they often will.