I would not put silicone over the old caulk. That will not hold. If you want to reseal it, you'll need to strip the inside clean.
I wouldn't even use a 10 year old tank as a display, that might be in my house for another 10 years, without resealing it.
If you take your time and do it right, it will not leak.
As for the transfer, I would cycle the new rock in a bin(s) with a heater and power head, and then do the swap all at once- break down the 75 into rubbermaid bins, and then swap out the tanks, re-aquascape, then add livestock.
I would thoroughly rinse the sand bed before adding it.