Unfortunately, when braces are fitted it will undoubtedly cause some discomfort for a brass player. There are a number of things that you can ask your dentist to do for you but these can often cause more problems than they solve.
Having wax fitted can cause issues with the space or distance between the teeth and lips. Air pockets may also develop and a weakness within the embouchure may well develop. Although the wax may help with any initial pain it may allow additional pressure to be applied giving a false sense of how to play.
The most important element to be aware of is that it is important to reduce the amount of pressure and pushing on to the lips as little as possible. The most important thing to remember is that preparation is key. The embouchure should be formed without the mouthpiece and the buzz should work with a clear, good sound. When the mouthpiece is applied it should be done so that a seal is made between the lips and the mouthpiece. Pushing the instrument on to the face should be avoided, as this will merely cause abrasion or cuts to the inside of the lips.
Check that the seal around the mouthpiece is uniform and that there are no gaps or any leaking of air, and that the angle of the instrument is correct. This should allow a clear, full sound with no hissing or excessive pain.Of course it is very important to take time over playing and complete the steps with patience. When the braces are fitted avoid trying to play all of the pieces or exercises that you used to. Work on things slowly and ensure that the foundations are laid before moving on. Follow things in a logical and ordered fashion as this will allow good techniques and habits to be formed. Time taken here will pay dividends later as the strength and technique develops.
Start playing in the middle and low registers, and work on long notes that are supported with good airflow. Remember to rest, and if you feel pain then try to adapt or alter what you are doing or simply stop. During the early stages it is important to play little and often and definitely avoid playing for sustained periods of time until the technique allows.
With dedicated focus and clear consistent practice there is absolutely no reason why the strength, dexterity and range should not come back within time.