Hey Andrew!
I'm Lara, aka silverstar. Welcome to the forum!
Ok, introductions aside...to address your question.
I just got my braces off in December. (After 3 years of them....it was about time!) Here's what I have found helpful...
1.) Take it one day at a time. Don't expect to be back to par for a while. Remember how long it took to adjust to your braces when they went on? It's about the same when they come off.
2.) Do a lot of warming up before you play. This helps to get your chops to remember what they need to be doing. If you try to play cold, your chops won't be your friends...trust me on that one!
3.) Lots of lip slurs and long tones. Try pedal tones too. If you have the "Advanced Lip Flexibilities" book by Chas Colin, play through the first page slowly and accurately.
4.) Talk to your teacher/director. They are usually rather understanding about braces and things like that. They will probably be the most helpful at your concert tomorrow.
5.) Keep taking lessons, and don't slack off! Slacking off is probably the worst thing you can do right now.
6.) Take breaks when you need to. If you're not feeling like things are working out and you've been praticing a lot...don't worry. A day's rest should clear things up.
7.) Free buzzing and mouthpiece work will strengthen your chops.
Ok...that's all I can think of for now. I hope it helps! About your concert tomorrow, just talk to your director. If you really feel that uncomfortable playing, you might want to see if there is someone who would be willing to play your part for you just this once. Talk to Van too...and if he threatens to stick you in a cage, he'll have to deal with me!
Welcome to the board!
Lara