Since the OP has not chosen to enlighten us with any information about why he has asked this question (come on, make that fourth post ) any advice is rather speculative.

Thee may be a peculiar reason that he has to actually purchase a premium economy ticket and upgrade on the day. Company travel policy? Or he is incredibly charming and buys tickets on the basis there is a 50% chance he will get an op-up but that he will pay for an upgrade if he doesn't get one?