VERY VERY CAREFULLY, lol!!!
Best thing would be to leave the gravel, and put the sand on top. . .
If you INSIST on taking the gravel out, get a pair or two of brand new pantyhose, and wash them VERY well in VINEGAR (no soap) and rinse rinse rinse. Then, take ol' Frogger out for a bit, and scoop all of the gravel into the toe of the pantyhose. Use at least 2 'feet', more if you can - but keep that gravel in there, and keep it wet. Then add your sand and fix your tank how you like it. Let the sand settle before turning the filter back on - and put as much of the old tank water back into the tank as you can. . . you want to keep as much as you possibly can the same - don't clean your filters or anything - skip this water change (unless parameters get funky - you'll want to test frequently, esp in a tank that size). Put your tank back together again, and put those ugly gravel-filled pantyhose back into the tank! If you have 2 pantyhose bags, wait 2 weeks - then remove the 1st one during the water change. Then wait another 2 weeks and remove the next. During your weekly water changes, take the bags out and gently squoosh them in the bucket of dirty tank water, to get any mulm out. If you have 4, remove one every week until they're all gone. The tank will look funky for a month, and be pretty cramped, but this is the best way that *I* know of to do this without making the cycle bumpy. . .
Maybe someone else can offer a better way! I'm SUPER cautious about this kind of thing, so I may be telling you the hard way, but I've done it, and it worked with no cycle flux - so I'm okay with recommending it :)
Last edited by Chesh; 07-15-2012 at 10:33 PM.