The PJ rubbers should be in good condition as the Hoofie rubber will then wear evenly
If you have metal showing through I would change them before you fit the Hoofies
Yes they should last longer than the PJ rubbers just make sure you fit them as shown with the holes in the right place then you can turn them round (It's obvious when you see them on) also keep an eye on them and when they wear right down to the first webbing turn them round (If you wear hoofs unevenly might be worth swapping left for right when they start to wear
So as the replacement rubbers cost £6 including p&p that makes £3 per wear so at £15 a set of PJ hoofs you can see the saving
The one thing I have found is the better you get the faster all types of hoofs wear
Jason