We did this on Sat 2/13 at MK. We had an 8:05 ADR at CRT with MK opening at 9:00. We arrived at the main ticket entrance at 7:15 and were first in line for the will call window (which is to the right). Will call opened at 7:30; we exchanged 1 voucher for a ticket which we immediately upgraded and also exchanged 1 voucher for FP for our group of 5. Everything took about 5-7 minutes.
Next we went to get in line for breakfast ADRs - we were 2nd in line. Everything went very smoothly and I was happy to have it all out of the way before we entered the park.