switching over livestock

[Aquattro]

As above, I would just transfer everything into the new tank. Take a few hours and you're done. Going smaller means you don't even need extra water.

[Palster]

Like they said you should see little or no cycle because the beneficial bacteria is already contained in the live rock and live sand that you are transferring over from the 90 gallon tank. I would even use the water from the 90 gallon which would save you from having to mix up a new batch of saltwater.