I still think that making them mandatory would be overly restrictive to deck building. Like what if I'm team life and I want to make an adreanastave deck or I want to use nova in my rush? I think if you're having an upped war, then you shouldn't impose any restrictions that would force suboptimal decks.
I also like malduk's point about the strategy that's involved in deciding what to up.
Perhaps something in the middle? Like all decks must use 15 upped cards?
The main problem here is that if you let people use unupped cards, we will have tons of participants with not that many upped cards. Players who do not pick unupped cards because those cards fit better to their decks, but because they don't have upped version of those specific cards.
What happens then?
Well, then we will have players with fully upped decks, fighting players will half-upped decks. When the latter lose, they will complain that they only lost because the opponent had upped cards and they didn't. The event would most likely be decided based on who has the most upped cards, not based on who has the most skill.
The playing field has to be fair, meaning that all teams must have access to exactly the same amount of upped cards. I don't see any way of accomplishing this, other than forcing players to use upped cards.