It looks like the 2 different terms are contradictory. We may have to wait and see if they clarify whether you just have to pay the extra fees or if you are completely switched to a paid account if you go over.
Of course, it would make sense if you only paid for the 'extra' charges if you went over. It would be kind of sneaky if they charged you for the whole thing if you went over.
I hope they do clarify this.