BE3600 Upgrade preparation

Hi folks,

My company has 1 CAS and 1 MMS BE3600 appliances in two different sites and I'm now working on the upgrade preparation from R2 to R3 and later to R4. I know from this article the recommended sequence. However, a couple of questions come up in my mind about it:

1) Now that there are duplicate jobs between the two appliances for cross-site data resiliency. During the upgrade, with the communication disabled, I know that all the duplicate jobs have to be put on hold. But after I finish upgrading the CAS to R3 while the MMS is still R2*, can I enable the communcation and unhold the jobs on each appliance to resume the duplication? Will the duplicate jobs encounter failure due to the different versions?

2) After I finish upgrading the CAS, can I upgrade the agents on the backup hosts (e.g. Windows agents, Linux RALUS) of the CAS's site first so that the local backup jobs can resume?

* The reason for me not to upgrade the MMS immediately after the CAS is because, first, they are located in two sites; second, I need to observe if the backup jobs will go wrong after the upgrade.

3) On the other hand, does the upgrade ever fail? I know I can back up the Data and Catalogs folders beforehand for emergency, but if anything really goes wrong, what should I do with these 2 folders? any specific guidelines for fallback?

Thanks.