I learned the instrument switch during WWII whereas as such we were losing from our band many High School seniors and some juniors who had enlisted in the armed forces. It was essential to keep a balanced band, even if fewer in number. Our band director had been lead cornetist with Sousa's second band and then went on to play with Ringling Brothers. My daily practice method has been 20 minutes playing, 20 minutes rest over a 2 hour period and that actually means just 1 hour playing and what you play can be anything, not to say methods are not good, just that attitude changes. I say that consistent practice develops and SUSTAINS stamina. There just isn't any instant attainment. As for the higher register ... altissimo ... unless you want to be an ostentacious show-off in the jazz genre seldom will such be a requirement for the bread & biutter gigs. No we are not Maynard, Arturo, Doc Severinson, Botti or Marsalis or other soloist ... but in time we will be able to play higher and higher with more and more ease. I still like my solo of Silent Night played on euphonium.