I have seen a couple of threads on this, likely search will bring them up.
I upgraded from a 125g to a 210g and it was a MAJOR undertaking - have plenty of help. My new tank was going into the same spot the old one was sitting. This was the big issue, much easier if not.
Let everyone know if the new tank is sitting in the same spot as the old, different advice will apply.
The sandbed will be filled with some nasty crud. I reused just a portion of it, sort of for seeding purposes. Do not know if this was the right decision. It did not seem to have any negative effects. Using much of it would have resulted in a huge crude transfer.
One thing that I did pick up on real quick was the need to have a couple of buckets available to dunk live rock in when removed from the original tank. The rock even though in current, sprayed off from time to time - still contained a lot of waste that was removed during a couple of dunks prior to transfer, make certain to do this.
