Monica is right. If you have a 2-bedroom reservation, you will automatically be upgraded to the Grand. If you are going using a 1-bedroom, you will be int one of the two 1-bedroom buildings. They plan to rebuild a big high-rise Mayan Palace where the current 2-bedroom building is. I guess they are building a Mayan Regency on the golf course. It will have a lazy river. I heard that the current water park will be reconfigured into a new pool in front of the Grand Mayan once the Regency is completed.
It would be a great time to use a 2-bedroom MP week if you have one - the MF are much lower on MP weeks than GM weeks. You get to stay at the Grand whichever one you use.
Acapulco is great. There is a lot to do, but we love just staying at the resort. It is our favorite destination/veg-out resort. It is ultimate R&R.