Permanently losing an upgrade point seems a bit steep; you could only use that one so many times before you’ve nerfed yourself into uselessness. Lose a few points out of offence/defence until the end of the forum maybe.

On principle I think the effects of a battle should end when the battle does… or at least be recoverable after the event. With that in mind, you could have “lose an upgrade point” as a cost, but only if there was a way to earn it back later.