well, it's not "much". a cycle is a cycle. when the tank is cycling, bacteria colonies grow and cover all available surface area. you removed a lot of the surface are when you changed the sand. you don't have enough bacteria covered rock to be able to compensate for something like that. all you have is those little pieces and the glass walls of the tank.
it's best to move the livestock to another tank (perhaps the lfs
would hold them for you for a couple weeks?), and then let the bacteria colony grow uninterrupted by water changes. if you do not and stick it out, the fish may or may not be ok through it. detectable ammonia is definitely stressful to the fish.