Welcome to FT! ^
As mentioned above, upgrade eligibility depends on status. E35 can only upgrade on a limited number of fare classes but E50 and up can upgrade on more.
You should also be aware the upgrades are subject to availability which in practical terms means there has to be R space. Do a search and you'll find lots on what that means.
By default the AC.com page will list the lowest fares currently available. So if you want to spend less, what you see on the page is what you can get at that point in time. Juggle your dates and you may see other available fares. Or learn how to use KVS or ExpertFlyer and you can see the fares there.