The live rock is OK but I wouldn't use more than a couple scoops of sand. The benificial stuff will eventually populate the new sand. If you move your sand over you will have a cycle in the new tank as all the trapped sediment is released. If you did move the sand, I would wait until the cycle ends before putting anything living in the new tank, and then you would have to be really carefull with anything living in the old tank after stirring it all up.
As long as it doesn't get dirty while draining it (IE: sand isn't kicked up into the water) then I don't see why you couldn't re-use some, if not most, of your existing water. You are going to need to make almost 100g anyway going from a 75 to a 170.