I have always re-used my sand. When I went from the 75 to the 180, put mostly new sand in though. I took the old sand out and washed it as best I could and it is now my 6-8" DSB
in my refugium. It did cycle but that was to be expected with the washing. I moved that sand 2x before with no problems. I did notice that it was pretty dirty when I tore it down this last time though. I usually try to keep a lot of nassarious snail to keep the sand stirred up a bit.
If you can afford it go new. If not, clean it up and don't be in a hurry ...
*180 gal Display, 100 gal basement sump, 33 gal refugium, 3x250 MH, 2x160 VHO actinics, zoos, some softies, LPS & lots of acros and other SPS.
*100 gal prop tank plumbed into main system w/ 2x96 PC lights and 1x150 MH,