Well, a year ago an international director allowed this at my table after 1NT - 2C, at our National Congress. The 2C bid was ruled unintended, changed to 2S. Another very experienced director in recent years has allowed my partner to change an unintended bid (I think 4H to 4S from memory, we had been bidding spades and the opposition hearts). I also have examples from workshops on Law 25 allowing changes with written bidding, such as changing a 4H opener to 4S.