The biggest impact we've ever had on our trips that coincided with Grad Nights (3 years in a row) is that they close the MK early. AND thereare BUSES AND BUSES AND BUSES headed for the MK. It was quite a lot of traffic. Thank heavens we were on the monorails from Epcot and on to the GF. They slip a letter under your door at the GF & Contemp warning that you may hear music & Noise late at night (we did not and we were in the building closest to the MK at the GF).
The convention centers at Yacht Club, GF, Coronado Springs, Contemp were used for gathering points for the grads as they arrive, but they get shuttled off in buses to the MK at the appropriate time. We've been at the other parks at this time so it did not impact us at all.