Hmm, that's the answer I expected, but not for the reason I expected. Maybe I'm not understanding the action change rules correctly, but I thought you got half of the dice based on the new action, which would allow you to switch from 0 dice to any action that gave you at least 4 before getting cut in half. I figured that a character using active defense would never actually get a "turn" in which they could switch their action.
To give the example from play:
My thug was facing 2 GM controlled enemies. I declared attacking enemy 1. Enemy 1 went for active defense while enemy 2 attacked. On the my turn, I decided to switch my attack to enemy 2 (even at half dice, I still had plenty to attack after pewter). The question was whether enemy 1 could now change his mind and attack instead (at the half-dice penalty).