Even on a small tank like yours you don't want to just pile everything into a 5g bucket. I say go to walmart and buy one of those totes they sell. I have 3 that I use for this purpose along with a 90g outdoor pond that I bring in and set next to my tank. If you have a larger container to put your rock in you can also put the fish in that container with a power head and a heater. Over a 6 hour period you water temp is going to be the biggest issue.
As Roka said try not to stir the old sand up too much as this can cause a spike in all kinds of nasty stuff. Also after you do the re-aquascape I suggest a PWC
and running some GAC
wouldn't hurt either.