Completely disregarding the issue of the written music (which others have answered well), my experience as a trumpeter has been that the embouchure is the bigger problem.

I had no problem adjusting to playing the Eb horn after only a few hours of practice. The bigger embouchure used means that I surely didn't get as fine a tone as an experienced players, but it was enough to get by with. Since then, I've doubled numerous times on Eb and the switch is now very easy for me.

The baritone was a lot more difficult for me. The mouthpiece felt huge to my lips, and I was only able to play for a short time before my chops become fatigued. Given some more time, I'm sure it would work out, but I would say that it's not obviously feasible with only a few hours to practice.

Tuba, I've only tried once, and a small Eb at that. Not gonna fly any time soon.

What brass band instruments can a cornet player feasibly cover on short notice?
Currently in Khojand, Tajikistan and wanting to take a taxi or something tomorrow to Dushanbe.  The taxi drivers here are like vultures - I was surrounded by the horde of them at the station outside town where the border taxi guy dropped me off.  I've been quoted US$40 for a ride to Dushanbe tomorrow, in a shared cab, or $100 for just me (I'd go shared) but it's in a regular car and have no way of knowing if that's good or not.  Googled without much luck.  I've been told you should only take a jeep, or 4wd, but everyone says different things (taxi drivers) and it's hard to know who to trust!

How it turned out - see answer at bottom, successfully made it to Dushanbe without being ripped off too badly ;)
