With any tank transfer, there is a potential for disaster. I have had one tank crash on me after transferring everything over. Since this is such a large volume upgrade, I would consider finding a place where you could hold your entire tank contents while you break down the 125 and install the 280. I just don’t see how you could do this quickly enough, otherwise. This is typically done with Rubbermaid / Tupperware containers and 55 gallon drums. But again, with such a large upgrade, I believe your stuff would be in these containers for too long (each one requiring a heater and powerhead). Therefore, you might want to see if you could borrow something large enough to hold ALL your livestock / rock while you upgrade. If you cannot borrow something, a 150 gallon Rubbermaid stock tank would probably be your cheapest holding tank.
The large temporary holding tank will now give you the time it will take to make all the extra water, position the new/washed sand, place any new (already cured) rock, and whatever else you need to do. Again, I just cannot see this transfer happening within a day, and a temporary tank will allow you to take your time and get it done right.
SAND: I would hold a cup or two of your current sand to seed the new tank. I would not use the remaining sand, unless it was completely cleaned.
STAND: The query concerning your stand cannot be answered, not knowing what you currently have. If I were to totally guess, I would say no, it can not be used. I would also be questioning the floor below the tank. If it is a slab, you are probably all right. If it is stick and frame, you will want to consider adding support below the floor.
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