Can Symantec DLP be upgraded in phases and thus uspports phased upgrades ?
It is possible with the V11 environment, but it has some important limitations.
The bottom line is: this allows you to spread out your upgrade over a few days, but it isn't practical to run your system spread across versions for any extended time.
1) Incidents are captured by "old" detection servers and are sent to Enforce, but no other info is passed between them. So you can't change policies on an "old" server, for example.
2) If an "old" server goes down for any reason, it can't come back up until it's upgraded.