We just went through this process with one of our 5220's and I'm thankful that we had a Tech do it because I couldn't find any good documentation of the physical parts of the process. Here's a quick overview of what we had to do:
1) Order a shelf and a RAM upgrade since we have one of the early 5220's that only had 48G RAM.
2) Rack the new shelf
3) Power down the Appliance (but not the existing shelf)
4) Remove the old RAM
5) Install the new RAM (We kept the existing SAS controller but the upgrade kit came with one if we hadn't had one already. We also reconfigured our Appliance for SAN fibre by taking out the 10G card and replacing it with two fibre cards but that's another project...)
6) Cable everything up but leave the Appliance unplugged from power (there is a good SAS cable diagram in the Shelf description PDF)
7) Power up the new shelf and wait for it to totally stabilize
8) Power up the Appliance and keep a close eye on it
9) Once it is fully up and the cache either starts to load (software 2.6) or is fully loaded (2.5 and below) then you can issue the commands to first "use the new area" then "expand into the new area".