JBossESB supports BPEL via the ActiveBPEL BPEL engine. The question you ask relates more to ActiveBPEL than JBossESB.
What you are describing is the concept of in-flight process migration. This capability is outside the boundaries of the WS-BPEL 2.0 standard and falls into the realm of each vendors BPEL runtime implmentation.
With that said, if are using the ActiveBPEL Enterprise editions then it does support the capability to migrate in-flight processes using a feature called Process Versioning. There are very specific rules in place to determine if an existing instance can be migrated. If your particular use case these rules would not allow the current process instances from being migrated becuase of the additional activity added (activity D).
Migration is feasible only when there are logic errors from an incorrect expression or data assignment, for example, an activity branch had an errant expression. If there are any structural changes to the BPEL definition than you are only allowed to do one of two things; one is to allow the previous instance to continue using old version or two actuall fault all previous instances.
Ah, I see.
Thanks for the terminology & elaboration.