I just upgraded (and still am) from a reefer 250 to a reefer 450.
I would throw away your old sand. It’s not worth the hassle of cleaning. You “could” use a bit of your old sand to “seed” but I’ve never seen any “proof“ that that’s needed. Too many quotes...
DO NOT, I repeat; DO NOT re use your sand without completely cleaning it and letting it completely dry out on a clean surface outside.
I also highly recommend going bare bottom, but thats up to you.
Get the new tank all up and running with RO water and check for leaks. Once your satisfied with zero leaks, you can add salt, a power head and a heater. Then you have a completely full tank with pre-mixed and heated saltwater.
Keep your old tank running the whole time.
Ensure the temp, Salinity, alk, and ph in the new tank are as close to the old tank as possible. This is why you always use a salt with the parameters you want to keep in your tank.
Now remove enough newly mixed and heated saltwater from the new tank to allow all your old stuff (rocks, fish, coral, equipment etc. and at least half (I recommend 75%) of the water from your old tank to transfer over. This is going to be your water change water in two days
Add your sand and let the tank settle...eww I hate sand.
Once the sand is settled It’s time to start moving. First add like 50% of the water from your old tank to the new and then move all your rocks coral and fish to the new tank. (Quickly). Don’t worry too much about the aquascape yet, just get things moved.
Add your equipment and then finish filling the tank from your old tanks water (do not pull out yucky looking water. Try your best not to disturb the sandbed (have I mentioned I hate sand)
Now, here is the most important part. At this point do not add anything new. No new rocks, no new fish, nothing! Wait! Be patient! Your fish and coral will be a bit “off” but to them it was just a big water change. Even though you just upgraded and have more water volume does not mean you need any more bio filtration because your bio load is the same. As long as you did a good job getting the new salt similar to yours you’ll be good. You will lose a bit of biofiltration from the old sand but it should be fine.
After two days, do another big water change (30-40% ish from your leftover water. (New tank, not old). Continue with 20% water changes every week for a month, the. Go back to your regular schedule.
Your tank will be a bit unstable for months as things find a new equilibrium. So go slow.
What I have been doing is adding a few more lbs of rock every month to cycle in the tank. If you go slow enough you won’t get an ugly faze. Use this time to scape the tank in your mind and think about how you want things to look when your done.
My transfer is about 3-4 months old now and since I’ve taken it slow and slowly moved and added things it’s gone well, but it could go horribly if your not careful. I just added a few more rocks, and I plan to add more in a month. Go slow!
I’m sure I forgot something, but that’s how I’ve been doing it