I was just at WDW with our high school band a couple of weeks ago and they also performed on the stage at Downtown Disney. They didn’t have costume changes so I don’t know how that will work for you. We had an excellent experience from start to finish. The people who were not involved were dropped off our bus about 90 minutes before the scheduled performance time. The bus then took the band across the street to the performing arts building to change and warm up and pick up their Disney rep. She guided them through everything. The band arrived at the stage, got settled and then played for about 15 minutes. We all then boarded the bus to return to the performing arts building to change back and take a few pictures. We said goodbye to our rep and left. It was all very well run.
Our band also did a clinic and made a DVD/CD that evening. The kids weren’t really looking forward to it before hand but every one of them came out of it smiling and talking about how much fun they had. The CD was set to animation and was very well done. We were all very pleased with all of it. Disney was very firm about how things had to be but that wasn’t a problem, they didn’t ask for anything but their high standards to be maintained.