Replication Manager OS upgrades

I have several production and mount hosts in my RM 5.5.1 environment that run Win2003 and SQL Server 2005.

We need to upgrade Win2003 to either 2008 or 2012, preferably 2012, while leaving SQL version the same.

We leverage the RM SQL agent today.

My question is: can we perform the OS upgrades one VM at a time or does RM require that we do all upgrades simultaneously?

Secondly, if RM does not allow different OS versions for source and target when using the SQL Agent functionality, could we at least simply replicate crash-consistent copies by changing the app set to Volumes instead of Database?

What have you all done when upgrading OSes on RM-related servers?

Thanks, gurus.