I have an SSR Taunton Reading ticket. I've used the outward portion already, but want to return on a train that is outside the SSR validity, but in the SVR validity, i.e 0935 from Reading. Can I change the ticket by paying half the difference between the two tickets? If so, what do I need to do, can I get on the train and ask the guard for an upgrade, or do I need to go to the ticket office first. What do I ask for exactly?