Script to modify Virtual Servers default pool?

Thanks for the explanation it is very useful. For the use case, yes I'm looking for change that is permanent so iControlREST should do the job. As for the active session I need to verify that too. Basically I would need the behavior should be like this: Once the default pool on the virtual server is changed all new sessions would go to the "new" default pool and all old sessions would remain and on the "old" default pool until they are being terminated from the client side.