I have 3 exchanger servers, with each server installed with CAS, HUB, Mailbox.
Ex2010A SP2 Ver. 14.2 (Build 247.5) Active DAG member (site A)
Ex2010B SP2 Ver. 14.2 (Build 247.5) Passive DAG member (site B) Active CAS server
Ex2010C SP3 Ver. 14.3 (Build 123.4) not part of DAG (Site A)
AD schema 47
Exchange forest schema 14734 (support 2010 SP3)
Total users 400
Obviously not properly design when all users are in site A but CAS is in site B. I am planning to upgrade my Exchange to all SP3 level and switch some roles.
Options 1: Add Ex2010C to DAG member and upgrade Ex2010A to Sp3. Move CAS from Ex2010B to Ex2010C and upgrade Ex2010B as well. ( I don't see any down time in this and look very simple, only issue new ex2010C has different SP. though I know from TechNet "You can safely move a database between servers running Exchange 2010 SP1, SP2 or SP3. Even though you can move databases between mailbox servers running different service pack levels)
Option 2: Make Ex2010B active and upgrade Ex2010A to SP3 and vise versa. Then add Ex2010C in a DAG ( only issue is since all users in site A and when I make Ex2010B (Site B) as primary there will latency issues which is I am afraid. Though upgrade does not take that long but what if its unsuccessful )
Option 3: Move mailboxes from Ex2010A to Ex2010C and move CAS from Exch2010B to Exch2010C. Once upgrade create a new DAG.
Any other ideas are much appreciated.