That is one way of looking at it, but I don't know that it would hold up in court. The TOC would likely argue that the passenger is still free to make as many journeys as they want within the validity of the ticket and I'm not sure a court would disagree.My view: purchase of a fare does not simply give the passenger the right to travel between the two points named on the ticket. Rather, it gives the right (to quote from the definition of fare in the TSA) to make one or more journeys on the Network. If the right to make some of these journeys (e.g. Shoreham to Leatherhead) was removed, then that is the material change. (And for anyone wondering: no, the TSA doesn't define journey!)
In any case the passenger is free to withdraw from the contract if the ticket no longer suits their needs.