We're in the process of planning our upgrade from SQL Server 2005 to 2008. Our 3rd party app is certified against 2008 so we do not expect any major problems, but we thought it would be best to upgrade all of our "subordinate" SQL Server instances to 2008 first in case we encountered some crazy error condition upon go live (we did a test import and all looks OK).
We have one 2005 instance in production with DB mirroring to another 2005 server. We also use transactional replication from the production to two additional standby instances (also 2005).
Can we upgrade the replication instances to 2008 without any problems? (my hunch is that we can, but I can't find any data to say this is 100% true)
Can we upgrade the DB mirror to 2008 without any problems? (My hunch is no, we cannot do this).
Finally, we would then upgrade the primary to 2008.