Adding a new ps6210 - can evacuation be done before updating firmware?

Good afternoon All

I have a pool of 2 PS62100 and I want to add a 3rd;  I have some very latency sensitive servers on my network and SAN.  Is it possible to add the new unit (if it has the latest firmware) to the existing 2 unit array (V7.1.5 firmware) and evacuate one of the existing units to the new unit so I can then update the V7x unit to the latest, then do the same for the 2nd V7x unit?   If I have to updgrade first it will take down the business for 2 x however long the updates take (stupid system !)

Any hints and tips gratefully received.

Thanks

Eric