yep, like folks said, you still need to do a cycle, even if it's a smaller one. i recently changed my gravel out (rough gravel was tearing up loach's barbels) and am constantly watching it for a mini-cycle, too, even though the original filter, sponge, decorations, tank are still there.
with anything new (whether it's a tank or a treatment for an illness), it's always best to monitor water quality before doing anything drastic, like adding a full stock of fish. for your new tank, you might want to add 1 or 2 hardy fish to produce ammonia to feed the bacteria and keep your biological filter alive. the fish won't be exposed to too much ammonia because your sponge will already have good bacteria on it to remove it, so it won't be totally cruel like a fish-in cycle. if the biological filter doesn't have any food (ammonia), it will die and you will have to start from square one with a complete cycle.
