There are changing rooms on the 5th & 6th floors in addition to the bathrooms. You can also use the stairs to go in-between both floors instead of waiting for the elevator.
Every other floor has a tiny bathroom. (For women - If you're on the 11th floor, go down to the 10th.)
A great warmup studio is right across the street - Michiko Rehearsal Studios, 149 West 46th Street.For non-eq folk - the closest bathroom is McDonald's.
