Hi Chris,
If it's new
sounds (and not necessarily a new
horn) you're after, why not investigate ways to get a more cornet-like sound from your trumpet?
For example, I've had great success with a Curry TF mouthpiece on both my B-flat and C trumpets. Check out Mark's description of his TC and TF model mouthpieces on his website (
Curry Precision Mouthpieces).
Also, have you exhausted the possibilities offered by various mutes (such as the Jo-Ral bucket or the EaZy bucket mute)?
Any of these things would cost much less (and be much more portable) than even an antique cornet...