I read your post a couple of times to see if I was missing this, but... will either of the tanks (old or new) contain ANY livestock at all? It could be okay if you're only moving live rock over and setting up a new tank that you can give plenty of time to cycle.

But, if there is any existing livestock in either tank, then BAD IDEA.

And when you think about it, in terms of the overall investment in either tank, a couple of bags of live sand is not that expensive. Way too risky to save a few bucks.