I agree completely that, up to my knowledge, this is a grey area in the rules. And I can fully understand and accept Rich's way of resolving the situation.
My own interpretation allows for more freedom to the heroes, and considers that the tides and movements of the fight allow other companions to close around the archer, or form a sort of man-wall, or whatever, and those in melee are able to engage the archer's enemy, thus freeing him to change stances without further maneuvers or skill tests.
But, as said, I think this is open to interpretations in the RAW, and each LM or table could decide for themselves how to solve it.