Decide on the new location of the piano, and plan a specific route to get there. You don't want to try to pick a spot when you're holding a heavy and unwieldy object. Make sure the new spot and the path are clear of debris.
Show your helpers the new spot, and plan a strategy. If necessary, measure the piano and any doorways to ensure that it will fit. Don't forget to consider the pedals and other protruding areas.
Move the dolly into position next to the piano, and add additional padding as necessary to protect the piano.
Carefully lift the piano onto the dolly, making sure it is balanced and the weight is evenly distributed. Keep a hand or two on it until you're sure it's secure.
Roll the piano to the new location, watching for doorways and other potentially bumpy areas. Lift the piano, and have someone move the dolly out of the way so you can gently set it in the new spot.