Question : Exchange SCR between versions

Have an Exchange 2007 SP1 update9 with mailbox, hub, uc roles & a seperate CAS server on the same build/patch level.
Also have just built Exchange 2007 Sp3 box to act as SCR at remote DR site. Can you do SCR between SP1 & SP3 or do they have to be at the same level.
Tried to upgrade the CAS server to SP3 but someone has done a disk cleanup on the C drive (I think I know who) and the upgrade cannot find the old exchangeserver.msi to remove the files prior to upgrade so will have to go with what I have got for now.

On the second point is there a way around the upgrade issue?

Answer : Exchange SCR between versions

yes, the servers can be different builds, however, if you failover to the scr node with sp3 you do not want to failback until you have updated the original source