I'm afraid I don't stand for any of this nonsense. If the train I'm booked on is cancelled, I always get on the next available service. I'm already delayed as it is. I'm not prepared to accept any further delay.
It is the TOC who cancelled the train who are in breach of contract and they need to recompense the TOC I actually travel with, not me. If there is no mechanism to do that, that's hardly my fault.
So far, ticket checkers have always taken a common-sense approach, but were I to encounter a jobsworth in this situation, I'd take the line that they need to take me to court if they want any extra fare from me. I'm pretty sure it wouldn't get that far as this is one of those matters where TOCs do not want to set any form of precedent.