Basically when you sign up for any game you agree to a license where they have all rights and you have none. There is really no recourse here as far as I know.
Barriss is still a useful character, and so is Asajj which came in the same package. The two of them have allowed me to win countless crystals in PvP and to advance in GW. So I don't regret buying the package, even though I still think the nerf is excessive and was done in poor taste (and stupid timing since it removed any interest I had in the new pack).