What is wrong with the existing part. board that it needs to be removed?
Can you go over it with 1/4" underlayment?
If you must remove the part.brd.,I would put down the 1/2" ply.The warranty on the flooring may require it.
My house was built before plywood was the norm also and my subfloor is 1X12s.They are gapped a 1/4" or so for expansion as are the 1x8s used for the roof sheating.Not an ideal surface for applying a finished floor so go with the 1/2" ply.Luck