It doesn't matter if its a P or D or whatever, domestic versions of both are normally an x-UP fare. You have a confirmed first class ticket... if the equipment changes you will be rebooked into first class. Its only business vs first due to the international continuation of the flights.
The only risk is as I mentioned... if the flight gets canceled then you take whats available or wait for a flight with F.

I think its been a while since United offered refund for fare drops also. Its standard practice to charge the $150 change fee and then you'd get a voucher for the remainder if it dropped more.