Welcome to FT!, JPMM

Upgrades can be nice but one needs to prepared to flight your purchased seat. The system to biased toward elites, those with UA status. PZ is the fare class you will need to see. PN is just for the highest UA status, GS.

When you request the upgrade, you can request PremPlus or Polaris or both. Even if waitlisted, you will be charge the most costly option you select. If your waitlist fails, you will be refund the appropriate amount after the flight. Waiting to book or not, probably will make little difference in the chance of clearing but the price of the ticket may change -- booking international 3 months out is a reasonable time to purchase.

Honestly for a non-elite in economy, your chances are not great. Perhaps consider purchasing PremPlus, which guarantees a PremPlus seat and will increase you chance of a Polaris upgrade.