Do you have any experience with VMware?
Build a temp infrastructure in the new location - depending on your workload you might want to dedicate hardware for the scanner and CC roles. Configure only the policies that are local domain or spam related.
Have your DNS provider change the TTL on your MX records to a low value to allow rapid propagation of any changes - say 1 hour. Do this 2x earlier than the current TTL to allow the TTL to propogate.
Just before you shut down in old location, set the new scanners to Accept/Hold and add them to your MX & SPF records.
Before shutting down the old boxes,
- generate any report you might need later - include PDF and CVS and mail them to your self.
- export any config data you can - Reputation config can be exported all at once (you don't need to do each section).
- screen shots of policy
- act on any quarantines, incident items.
Move to the new site.
You can try Clear OSconfig, but I'm betting that you'll need to rebuild the moved boxes.
Build/Validate configuration, policies, changes to per scanner settings: DNS/NTP/next hop/IPs allowed to connect, etc.
Once you get the hardware moved/configred, drop the held mail from the VMs onto your moved SBG boxes.
BTW: I've been able to get SBG running within the free VMware Player (32 bit RedHat Enterprise), build from ISO, but I'd not recommend this for any significant load.