I did mine sections at a time.
For some of the sand, siphoning it out as part of water change worked really well.
Just use a larger hose.
My problem was that big chunks of the old sand bed had turned to concrete. I had to use
a chisel-like rod to break them out, very carefully without demolishing the tank.
For new sand, the key was washing it really well before using it.
I use a bucket with the garden hose / nozzle. Jet of water to stir it, and tilt
the bucket so light cloudy crud washes out. Clears fast.
Then a simple rinse at the end in salt water before using.
That said, I rather regret using sand. Looks nice, but always such a pain.
Future plans will be mostly fake-live-rock bottom, with some crushed coral and just a bit
of fine sand.