UA does not offer any such product. You can upgrade any fare class (except Basic Economy, which isn't offered internationally yet) with a Miles + Copay upgrade. The copay is higher for lower fare classes -- but the copay is returned if the upgrade doesn't clear. As another poster said, you may need to nudge UA to remind them to refund it.

OP: You'll get a display during the upgrade process that will tell you whether or not some number of seats are currently available. If they are, your upgrade will clear immediately. If they are not, your upgrade will be waitlisted and may clear, but the waitlist is ordered by status and it's nowhere near a sure thing.