I had an extremely successful embouchure change at the age of 15, after I'd been playing for 3 years. This was done at the urging of my teacher, the late Delbert A. Dale. (Anyone out there who knew him?) I'd been playing with a spread-out, puckered embouchure that had the rim contacting the center of the red part of my upper lip. The result of this was that I had poor range (inconsistent high C) and very poor flexibility, since I used substantial pressure even in the mid-register. (If you're already pulling back hard in the midrange, how can you hope to reach the upper register?)
Mr. Dale told me something I'll never forget:
"I blame world wars on bad embouchures. It's the single most important aspect of trumpet-playing, and without a good embouchure, no amount of practice will help you. The way you play now, you'll succeed in your high school band, but *never* as a serious player, and certainly never have what it takes to play lead in a commercial situation. [He expressed disdain for non-symphony playing, but knew that's where I was leaning.] Your range will forever suffer, as will your tone, flexibility, and endurance. If you intend to continue after high school, you'll have to fix your face. Otherwise, take up the sax."
Dale asked me if I could buzz without a mouthpiece, which I could do quite clearly (*not* like a raspberry sound). He then observed that this was nothing like my embouchure. He asked me to buzz, then bring the mouthpiece up to my lips without interrupting the sound. This was quite difficult, since I'd never played this way, but after a couple of weeks experimentation, something clicked and my playing changed to an amazing degree -- I was instantly using about 50% less pressure, and as a result was able to hit high f's and g's, as well as do lip trills and wide shakes that had been unattainable. This was quite a revelation for a 10th-grader. Dale was astonished,and told me it was the best embouchure change he'd seen in 20 years, and gave me a free lesson. Of course, I hadn't practiced my Clark book and promptly wasted the lesson :-) About 6 months later, another player in my high school section did a similar change with even better results -- consistent high g's and great flexibility. (damn him)
Since we were now using less pressure, it took only the slightest increase in air flow (or pressure) to slur upwards. Another effect of this embouchure was resiliance -- we bounced back extremely quickly after becoming fatigued during a long passage. These days, my range is closer to a double-c (not that there are many gigs that require that note, much less make use of it) and I find that I am able to commit atrocities such as not practicing for many days in a row and still bounce back with just a couple hours of practice spread out over 3 or 4 days. Unfortunate, but the embouchure lets me get away with it. My endurance suffers from a long layoff, but not my range, and the only thing between me and endurance again is a sufficient amount of practice time.
I've become convinced that an embouchure change can be a great help in one's playing. There is another testimonial to this on Donovan Bankhead's site, by a player whose name is (I believe) Eddie Lewis. I would have provided a link to this, but it appears Bankhead's site is down. However, you can find it via the Maynard Ferguson site.
No disrespect to the many fine teachers out there who advise against embouchure changes, but I have seen many examples of people who have never before played a trumpet, but picked one up and played double-g's and even c's since they were playing with good embouchures. They didn't have a lot of endurance, but the notes were clear and healthy, not strained. On the other hand, I see players every day who practice many hours a day, and have great difficulty hitting a high c or d consistently.
I also notice many similarities in the embouchures of great lead players. Check out footage of Mayard, Arturo, Roger Ingram, pictures of Bud Brisbois, Dave Stahl, Stan Mark, Scott Englebright, Doc, Chuck Findley, Wayne Bergeron, Bill Chase. You'll see very similar embouchures. At least I do.
Hope this helps someone.