TAD,
This is the procedure I did when moving from a smaller tank to a larger one.
Assuming you are not going to set up the larger tank and let it cycle on it's own, it's possible to swap out the smaller tank for the larger one and not lose any of your fish/inverts. Not being able to have both tanks set up at the same time is unfortunate, but should not prevent you from making the switch.
One very important thing I feel you MUST do when switching to larger tank is to keep your current biofiltration and live rock from your 35 hex wet, warm and the water well circulated in whatever container you use.
I made a switch similar to you question couple years ago, - going from a 55 to a 75.
I was changing over from an old aragonite sand, CC, mixed shell substrate, to a new 4-6 inch DSB.
I used two 29 and one 50 gallon rubbermaid containers. Put all my fish, inverts and live rock in the two smaller containers, with heater, powerheads and what few bioballs I had in the sump ( any filter media you have should be kept wet in the warm tubs as well ). No corals were in this switch.
Made the switch and used about 80% new mixed saltwater - and only 20% from the existing tank. I had a nitrate issue in the old water, so I didn't want to keep all my old water. I would never suggest making this large a water change to another person.
I did it - it worked fine - and was done with great caution.
I set up the new tank and added new aragonite sand, and filled it with the mixed saltwater that had been running in the $14.00 50 gallon rubbermaid container, with powerhead for 2 days prior. Added heaters into the tank at this stage. Added a couple powerheads for water movement - temp and salinity finally stablized after several hours - then added some of the live rock, about 75% of it. Left the rest in the tub with fish.
After the cloudiness of the tank water settled a bit, checked and adjusted salinity again, and checked pH to match as close to old water as possible.
Hour later I added my filtration back ( sump with wet/dry and handful of bioballs - you could add what ever additioanly filtration you may have on your new tank at this time - hopefully you've kept your biomedia/filter wet/warm and circulating all this time in the tub ).
All in all - it took about 7-8 hours to make this switch - not including clean up afterwards.
Could have been less if I had some help.
I left the fish in the tub overnight with heater and 2 powerheads, one low in the tub, one place higher for surface water movement, and few chunks of live rock to hide in. Checked them often, they were fine.
Next day I slowly acclimated the fish and inverts into the new 75 as you would normally do after purchase. Added the balance of the live rock that was in the tubs, took 2 fish back to lfs for credit that were no longer in the tank plans.
Anyways - sorry for the long post - but that's about all I can offer.
Hopefully your tank swapping will go well .... take your time, but don't waste time. Keep your biofilter running - don't lose your bacteria.
