Greetings UA flyertalkers!

I'm thinking of comping to UA Premier from NW silver elite and I want to better understand UA's upgrade policies.

On NW, if there is "I" bucket availability in business class (there is a galileo internet link for this), you can buy a Y or B fare and then use 50K miles to upgrade to WBC and have your upgrade confirmed at the same time you buy the Y or B fare. This way, you don't end buying a high fare but getting stuck in economy (which sucks big time on NW). This policy is open to even silver elites.

From checking the UA posts, I'm kinda of confused. Can you guys determine UA's C upgrade availability, buy a H fare, use miles and confirm the upgrade at the time you buy the ticket? Or do you always have to wait until 72 - 24 hours until your flight to know if your upgrade cleared? If your upgrade didn't clear anyway to refund the H fare?

Sorry if this has been talked about before, but I couldn't find an exact post on this matter. Thanks in advance.