The lawyer-ish response to that would be that such a change could be considered an admission that, at the time of the original campaign, the T&Cs weren't clear

The lawyer-ish response to that would be that such a change could be considered an admission that, at the time of the original campaign, the T&Cs weren't clear on this matter and that some people therefore had expectations of a guaranteed return on their pledge. It doesn't matter what the T&Cs say now.