I am now in my 3rd year of an embouchure change. Well, actually, I've been playing for about 3 years (my second trumpethood) and I have been searching and learning for that long. I started again after 30 years when my high school alumni band contacted me about playing for a parade. I figured it'd be fun. I had not played since high school. Little did I realize just how much I missed playing the horn. I have not stopped playing since. I practice about 360 days out of the year.
Many things came back to me once I started playing again. However, there were many that didn't too. One of the little things that bugged me was going from B to Bb. There were three notes instead of two! Some of the finger dexterity/coordination was missing. Practicing slowly to do it correctly was the only way to get it back. The chops? Of course they were gone. But also gone was my ability to read music. Sure, I knew which buttons to push but reading syncopated rhythms and accidentals required slow deliberate practice in order to get them back. Breaking the beats down into smaller intervals help with this ( e.g.: 1-and-2-and- ... instead of 1,2...). Another help was just slowly reading through a pattern until I understood it. This allowed me to interpret the pattern the next time I saw it instead of trying to read each individual note again. Playing duets with a friend who was a good reader was perhaps the most effective way for me to re-learn how to read the music. Lots of fun too.
One pitfall that I think is very difficult to avoid for a comeback player (it was for me) is trying to play what you used to, the way you used to. I believe that without proper practice and conditioning it will invariably lead to bad habits and poor technique. Range is one example that is perhaps the most pervasive and damaging. If a comeback player tries to play the high notes they used to, without developing the required technique and the moderate strength required, the result can be development of a poor embouchure. Unnecessary pressure is one frequent result. A poor embouchure not only affects the high range but the entire range of the player. Their sound and flexibility can be inhibited. I believe that the pursuit of range (both high and low) is necessary to development of a good embouchure. Let's face it, every one works on range to some degree. Think back to when we first learned to play. We played on an embouchure that allowed us to play our range (in the staff) comfortably. Not with great flexibility, but comfortably. Then, as we expanded our range we needed to make our embouchure work with increased flexibility over our comfortable playing range. Extending our range on a single embouchure setting necessitates adjustment of the embouchure for each note which adds flexibility. This must be done without excessive pressure and tension though.
Pressure has been one of the biggest impediments to my playing as I'd like. I learned to play with (as I realize now) a smile embouchure. I therefore am sure that I was a pressure player in high school. Examination of my bottom teeth confirms this. They are straight and flat across the front. When I started again, that is the way I started to play; with pressure. It was not until I started working on both the low (pedal) and high range that I finally got the feel of how it should be. That is a search that must be made by each individual. To find the way to incorporate both high and low range into one setting and type of embouchure without resorting to pressure. Teachers can tell you how but you must ultimately learn it for yourself. Clyde Hunt's book "Sail the Seven C's" is the book I credit for showing me how it can be done and how it feels.
Another problem I had (and comeback players in general I believe) is practice. A comeback player is most often someone who has another life. That is, they have a vocation and possibly other avocations besides the trumpet. The pressures finding time for family, of car repairs, house repairs, and job pressures all compete for our time. Finding the time to practice is not always easy. I tended to fill up what usually ended up being 1 hour's practice time as completely as I could. In so doing, I developed some improper habits that were tough to break. Mouthpiece pressure, sloppy technique, and poor tonguing are all problems that can, and did, arise from craming too much practice into an hour. A better approach is to select exercises that will work on a particular aspect and make sure that you concentrate on that aspect as you practice is. Rest is also important. Some say rest as much as you play. I had thought that by doing this you would not improve your range or endurance. But these are better approached seperately. Resting during practice does not prevent you from attaining these things but it does allow you to learn how to play properly. Playing on tired chops only develops tired ways of playing. How you play is how you have practiced. Learning to play properly (minimizing pressure and adjusting the entire chops for each note) will allow you to attain the high range and will give endurance. This was a hard lesson for me to learn but it is very true.
Another concept that was amazing to me the first time it learned how to do it was centering the tone. Horns are designed to resonate. That is what gives a trumpet a sound different than the buzz we get on the mouthpiece. If that resonance is optimized, the result is that playing is much easier, volume is much easier to obtain, the high range is easier, you can color your tone, and your sound will be very rich. The sound almost grabs the listener by the collar and say "LISTEN!". In the playing in high school, I had never experienced this. It was a revelation. IMO it is a very important aspect of playing the high range. Playing flat (often the case with me) makes playing high notes difficult which leads to pressure and/or tension (throat and oral cavity). These in turn make playing difficult which leads you to more of the same. Tension is now one of the biggest problems I deal with. It can, in essence, feed on itself (tension leads to more tension in order to overcome the problem of not being able to play because of tension) with the result that playing just above the staff with good tone and flexibility can sometimes be difficult. These times don't happen often but are very frustrating when they do. Usually they are a result of trying too hard, repeatedly, to play high notes. It is an easy challenge to get caught up in.
I've read all the suggestions about embouchure and how to practice, yaddita, yaddita, yaddita... I knew I should pay attention but, hey I'm an independent kinda guy. I can find my own way. (He said boldly; if ignorantly.) Well, maybe. Then again, perhaps I should pay attention. From my perspective the common elements that we need to pay attention to when we learn to play, or when we change an embouchure, both of which apply to comeback players, include:
Praticing without resting will lead you to abandon those subtleties for brute force. It is hard to make a subtle movement with a highly fatigued muscle. Try running up stairs for a few minutes and then stand with your knees slightly bent. Not easy and if you are not used to doing this kind of exercise, your legs will actually shake. Rest and it's easy. What you are trying to learn is subtle control. Not easy on muscles fatigued with overwork. Endurance is another issue that can be addressed separately but is also affected by how efficiently you play. If you learn to play while you are tired (no rest) and develop bad habits, that is the way you will always play. Learn to play correctly, and only correctly (with rest so you don't resort to behavior made necessary with fatigued muscles). Then, when you play, you will always play that way.
One final note. Over-analysis is, for me, a problem. I know that playing can be easy, including the high range. But, whenever I try to analyze the details of my embouchure, it usually leads to a sort of paralysis and tension that kills my ability to play over a wide range (low to high). This is prehaps not true of everyone, but I find that if I focus on finding a way to manipulate my chops (oral cavity and lips) to make playing a note easy, it is much more productive than getting caught up in the detailed workings of something I can't see anyway. The downside is that when I have a problem, it is often difficult to solve. However, using the same approach (ease) is almost always the best way out.