1. There is no ‘out of the box’ means to disable “Plan of Record.”
2. Processes on Cost Plans don’t appear to have the feature which allows one to start process based on current vs. previous value – this is probably why you asked the question, yes?
i. On Cost Plan object, Audit Trail, enable “Plan of Record” as an audited attribute
ii. Build your process
Note: I’m looking at 15.2 – haven’t seen 15.3 or 15.4, therefore I don’t know if anything has been changed/improved in this area.
Dale