I will be doing the same thing you are, but I'm moving from a 50 to a 120.
I think it really depends on what kind of sand bed you have. If it's a deep one, then it may be a little more tricky because when you stir up the sand bed (depending on it's depth) you may be stiring up alot of nitrates etc. IMO, you should use all your current water, plus a batch of new airated water. This way it wont be quite as shocking to the inhabitants as brand new water would be.
My sand bed is actually quite shallow, it's only about an inch to two inches deep, but I do have a DSB in my fuge.
GL with the move, and I can tell you, you will want more than that 50 very soon
Edit: I didnt say what I wanted to heh. You could start taking out some of your sand now, and slowly decrease your ammount over a period of time. You could keep the sand in a rubbermaid container with a powerhead in it to keep things circulating and if you have one, maybe a home made skimmer. This way, it might difuse the transition of the nitrates. I may be wrong, IMO this is the way I would do it.