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I'm getting a black rhom on wednesday this week for my 75g, but I still have to get rid of the RD I have in there now. Due to time constraints I dont know if I could get rid of it on Wednesday. Could I get rid of it on Tuesday. Would that be too long of a time w/o an ammonia supply to have the bacteria start dying off. I am thinking of adding an AC 500 from another tank to help.

I couldn't imagine that one day without an ammonia supply cause a mini-cycle. I could be wrong but I just don't see that happening.

-Josh

Dirt in gravel could potentially supply the tank with decompositional ammonia for many days, I shouldnt worry about it.

Bacteria start dying off within about 1 day of no ammonia supply. Thus the reason why when we fishless cycle we have to add the ammonia daily unless you add so much ammonia that it takes the bacteria several days to multiply enough to use it all up. That high of amount can slow the population growth of the bacteria.
Current fish waste and excess food in the tank will probably keep it at least partially cycled but you could always just add a little food like you were feeding the fish that is no longer there. Whether it's straight fish food or the waste from the fish eating the food it will still break down to create ammonia. Occasionally people fishless cycle tanks just by adding fish food every day but it's quite a bit messier and harder to measure than using pure liquid ammonia.

I've had tanks sit empty for 24 hours between fish with no problem -- that is, no ammonia spike when adding the new ones.

I haven't tried going longer than that, though. If the tank were going to be empty longer than 24 hours, I'd probably add Biospira with the new fish, and keep a close eye on things.