Before the Cavalier difficulty, before the 6th act, where there was no requirement for champions, you could upgrade the top champions and play at any time.
There are certain champions needed everywhere right now. In Variants, in Act 6, in monthly Cavalier. And if the champion is on AQ/AW, that's it, you're standing. You can't play.
Champions cannot be unlocked even for units.
@Kabam Miike , do you think this is right?