Is it possible...? Anything is, but you're certainly going to be in an uphill battle from the beginning. Out of curiosity do you have a flute player you could try on Tuba? Flute players can often make the transition better.

It's also early enough I wouldn't give up on the low brass kids being able to transition myself. Even if you have an early date, that's a few months to work past an initial block.

If you can't have solid lows though I probably would skip contest this year.