Thats odd. For the last 3 years we have taken a LATER bus with no issue. It would never have occurred to me to call and change our "reservation." We got back to the hotel when we got back and just got on the next bus. They have never asked for any type of paperwork or anything. It was from the Wilderness Lodge and in 2007 Riverside.
Our first year with ME we took the "assigned" bus and got to the airport ridiculously early. We actually had a full sit down meal at a restaurant in the airport while waiting for our plane. We vowed then and there to not waste precious park time sitting in the airport ever again and we have never had a problem. In fact last year I even stopped at the
Disneystore in the airport to see if they had the last pin of a set I was trading for. The manager even went into the back to see if they had it in the trading box. They did not, but I had plenty of time to do that and I got on a later bus.
But, i might add we drop off our luggage to ME first thing in the morning before heading to the park, so our bags have plenty of time to get on the right plane home. i think that makes a big difference since they go to the airport soon after you give them to ME (found that out the hard way).