Definitely put the rock into a rubbermaid tub with powerheads and livestock into tubs or buckets with powerheads... these tubs/buckets should be filled with water from the tank..
then scoop all the sand out while keeping it underwater over to the 125 and put it in the tank and add the rest of your new sand to it while mixing it in, it is important to note that you should only keep about 1-2inches of water above the sand in the new tank so that it settles quickly.. after it has settled, put a dinner plate on top of the sand so that you can start filling it with some new tank water very slowly just to bring up the water column height to about 7 inches. Then transport all live rock over and add the water that was in the liverock tub.. be very careful as to not stir the sand up too much. After this has been done and the additional new established liverock has been added (Do not buy uncured or newly cured rock as you may start another cycle in the tank) and the tank has run with powerheads for at least 24hours, you can add the fish (provided the tank isn't showing ammonia or nitrites) and the rest of the water... if you need to fill the tank with anymore new tank water, do so at this time...
I hope this helps.. most deaths occur if livestock is added immediately to a tank with a stirred up established sandbed so letting the sand settle back down is VERY important as well as keeping the liverock and sand under water at all times.
