fg18 - Yay another advocate of ergonomic alterations! whoohoo!
Back to the topic... I had to relearn my embouchure once only, thankfully. My first few teachers never said much about embouchure, usually just on more of how to get the sound out, and they tutored in groups in the school band programme so they didn't have time to pay attention to me. I didn't have any lessons for about three years. I was in one of the music programmes in my school (I'm from Singapore so it'd be a bit lengthy to explain it) and it was required that I recieved regular instrumental lessons. Having stopped piano lessons and swapped my primary instrument to flute, I went to get a flute teacher.
The first thing my teacher did was to correct posture and embouchure. He taught me an extremely relaxed embouchure, which allowed me to develop embouchure flexibility. With that, I could manipulate my embouchure in any way I wanted to, so effectively, I can use any viable embouchure with proper practice, i.e. getting the desired timbre, focusing the tone and developing endurance is another matter altogether.
It took me a while to master using a relaxed embouchure, though, say, half a year of daily intensive practice and a lot of frustration and pain. Some nights, I'd spend half an hour developing the tone of just one or two notes, playing just that and nothing else, and without much results. I was thinking about it, and I figured that flute playing is very much like body building: you may not see immediate improvement, but under that layer of skin, your muscles are developing. So don't worry if it takes a while! it's all part of the process.
I heard that all flautists suffer these "bad embouchure days". I recently heard a pro practising and she somehow couldn't squeeze the low notes out AT ALL! Robert Dick offers his "throat tuning" as one solution to such problems, but sometimes it's just that our chops are too tired or still in a groggy state.
Also, try to avoid "cursed spots" where the acoustics of the surroundings make you sound bad; it is really very depressing to suddenly "lose your sound" for no apparent reason. If you're feeling down about your sound (for some apparent reason), perhaps you could just play in some ridiculously echoey place, e.g. a stairwell, to make yourself feel better
The flute family: probing the lower limit of human hearing and the upper limit of human tolerance.