Rules are all very well . . .
Interestingly, a number of people at the Umpire Seminar, had to have it explained how a boat could go to windward by backing a sail. These were experienced people. So one can assume that the average protest committee member is going to find the concept equally challenging.
My advice is that, should you be caught on the start line by a boat that comes in under you, shoves the sail out (backing it) and then comes to windward and hits you, Don't protest! If he protests take your turns. You are not going to win this one in a protest room. OK if an umpire is present the rule can work but forget it for a normal protest.
You cam talk all you like about his action causing a change of rights - so no transitional arrangements - but protest committees will decide on the balance of probability. How are you going to persuade a committee that it was probable that a boat moved to windward as against moving to leeward. Itisn't natural, bow could it happen? You were windward boat and failed to keep clear - end of story.