
If you go to the ticket office before making the return trip, reckon that you
should be able to excess up to what the price of a "Saver" (Off-Peak) return would have been without the railcard discount.
If you get gripped on the train, you will almost certainly have to pay the full standard fare, which will undoubtedly be a lot more than a "Saver" return.
P.S. Depending on when you are travelling, don't forget also that with certain TOCs (such as Virgin) the arrangement you may previously have enjoyed, by which 16-25 Railcard holders can travel on peak trains with a (16-25 railcard discounted) "Saver" (Off-Peak) ticket will no longer apply, now that you no longer have a valid 16-25 Railcard.