I'm flying EZE-IAH-SFO in August. The fares on the CO site are $865 for economy and $6950 for business. I'm Silver and have about 100k ffm. The 865 fare is non-upgradeable, but the last few times I flew the route I never failed to get a complimentary upgrade on the IAH-SFO leg both ways.

Question: is there a way to book a flight on the site or on the phone that gives me a 100% chance of upgrading with miles or am I stuck gambling on an upgradeable fee that's 3x-4x the lowest one for an uncertain probability of upgrades? If I have to gamble, is there any way to know the odds of getting an upgrade?