Hi Mike,
If you transfer the rock and sand in such a way that you don't kill off the bacteria (e.g. keep them wet) I would be surprised if you get any sort of cycle at all. The key is to be well prepared and make the transfer as quickly as possible. If the new water to make up the difference in size is aged and not freshly made up this will help stabilize the water parameters. I've found the large green garbage pails are a cheap form of storage for this type of venture.
Regarding the fish in the holding tank, it sounds not much different from the tanks of 15 years ago before we got all fancy with equipment. You are not confing them to a much smaller space so I don't see any reason why they wouldn't survive the transfer providing they don't jump out from fright.
Planning ahead will make a big difference in how this goes for you at the end of the day...good luck
/Paul
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