Amy,
I upgraded from a 39g sumpless tank to a 72g w/ 20g sump. Almost a 50g diffrence, which is what your upgrade would be, around a 50g diffrence. I didnt let the new tank cycle at all. I really didnt have much choice as my old tank was about to burst, but everything worked out ok for me. I never got a cycle in the new tank. I used new (dry, bagged)sand and got a couple pounds of live sand to seed with, from diffrent peoples tanks. I believe the MOST important part of it was using all established rock. When I was doing my upgrade, ScottyC1 was taking his 72 down and I bought all his LR that had been in his tank over a year. It was all fully cured with nothing that would go thru die off, producing ammonia.
In your case though, you have quite a bit of established LR already in your current tank.. I would just swap everything(except sand) over to the new tank straight from the old one. Than leave your new tank running for a month or better without adding any other livestock or increasing your bio-load in any way until the new sand bed matures. Stay faithful with weekly water changes atleast for the first month, and check your water parameters often(daily or every other day at first) to make sure you dont have any cycle. If you do, than water change, water change, and more water changes.... remember, "Dilution is the solution".

Also be sure to use every salvagable drop of water from your current system, than add freshly made water to that for your extra 50g. That way you'll have established rock and water running thru the system from the get go, with lots of beneficial bacterias. If you have any HOB filters or canisters on the 75 , use them on the new tank for the first couple weeks also to help promote bacteria growth.
That is all I did and never had any signs of a cycle or a mini cycle even. I didnt lose any livestock either....Hopefully the same will hold true for when I haveta move the tank this June :hmm2:
Either way you go about it, Good Luck and send me a PM if ya need any help/advice with it.
