If it were me and I was planning to upgrade to a bigger tank, I'd just take the rock out of the old tank and put it in a tub/trashcan with some clean water, a heater and some powerheads while setting up the new system. I'd be sure to swish the rock around or blow it off with a powerhead as I was pulling it out of the old tank to get rid of any detritus and decaying gunk.
At that point I'd break down the old tank and salvage any equipment I needed from it to setup the new tank. Once I had the new system up and running I'd add the rock. Personally, I would not try to keep much (if any) of the old sand as it could be a nutrient bomb by now. If it looks devoid of life I'd probably just trash it. If you do keep it, I'd at least try to get any detritus out of it by stirring, siphoning, etc...
JMO & HTH