Um. Not sure if you are asking about switching to JUST large bore? Or did you want to continue playing both horn sizes? I think that would make a difference in your approach.
If you are going to stop playing small bore period and switch to large:
- The best way to do it is to up and do it
. Stop playing the small bore until you get a handle on the larger horn. From personal experience, it will not be difficult to return to a small bore in the future if you choose to do so.
- As BGuttman says, you need more air and an embouchure adjustment when moving to a large horn. Until you get the hang of pushing more air through the horn, endurance will suffer. Until you learn to adjust to the partials and range challenges of a larger horn, flexibility will suffer. This is normal and temporary as you learn your way around. Persistence will get you past it.
- Listen to recordings of large bore trombones to get an idea of what you want to sound like. Work to emulate that.
- Play some of the things you've enjoyed playing on the small bore. Feel and hear the difference. Don't let the difference bother you or frustrate you. It will take time to get the sound and the range that you want.
In addition to the above, if you want to continue playing both small and large bore horns:
- As bonenick said, spend time on both every time you practice. At least equal time on both, and extra time on the larger if you need it. I'd suggest playing the same scales/patterns on both horns every time you play them. Doing this will show you differences, and adjustments that you'll need to make to sound good on both instruments.
Overall, be patient. It's a big adjustment and doesn't (usually) happen quickly or overnight. I learned bass trombone in college and was very frustrated, hated the bass for almost a year. In the second year, after really buckling down and spending quality time on the bass, I finally got the hang of it to the point where I was able to gig with it.
Sorry for the long answer, unless it helps! Good luck!