Is it possible to delete a Cost plan with Plan of Record (POR)?
Yes, from CA PPM 14.3 Version its possible to delete Cost plan with Plan of Record (POR) and allowing you to have zero cost plans within the project.
This is a new feature mentioned in 'Change Impact and Upgrade' Guide, unfortunately for older versions is not possible to delete it.
For older versions, create a new cost plan within the same project, mark the new one as POR and then you can delete the first cost plan because it is not a POR. You will still be left with at least 1 cost plan marked as a POR in the older versions.
For more details see 'Change Impact and Upgrade' Guide from CA PPM DocOps.
Ability to Delete all Cost Plans. You can now delete all detailed cost plans including the plan of record (POR). If you delete all your cost plans, you can use the financial summary page to define cost planning data for your investments. Deleting the POR or other cost plans does not delete any associated budget plans.