The old passport number is mentioned in the new passport. See if there is a number mentioned. If there is, then you should send the old passport as well but for filling the application you'll obviously enter the current passport. If it says "N/A" in the Old Passport coloumn in the new PP, then you dont have to send the old one.
My OLD PP was made back in 2004 when Pakistani PPs were hand writted. When I made the Computerized PP, I didnt have any record of the old one. My new PP says "N/A" at that column and so I just submitted one. Whereas a friend of mine had a PP Number mentioned so he sent his Old Passport too.
We both are done with our medicals so far!