If F seats are available, you will receive an offer during OLCI - it will depend on the route and how many seats are available..

Offer 1 - a buy-up. This is an offer to pay the difference between your paid fare and the lowest published F fare that was available when you bought your ticket. As a UA elite, the lowest fare for you would be Z

Offer 2 - a segment fee (rare). This is an offer to pay a flat fee to upgrade your flight and depends on the number of open seats and the length of flight among other factors. This offer only comes up if the number of unsold F seats exceeds the number of CO elites waiting for upgrades

Your UA status has no bearing on which of these offers you receive, the price or whether or not you will even receive an offer at all.

If you must upgrade, call CO and request your ticket be re-priced as a Y or B fare (there should be no change fee) and then redeem a *A upgrade award if seats are open.