I can confirm Wallsendmag's advice that VT East Coast will sell an 'excess' to remove the Railcard discount and this is, in practice, offered to passengers at East Coast stations without hesitation.
However, I feel I should give some cautionary advice to people who might wish to try this when they hold a Railcard discounted ticket and will be unable to produce the appropriate Railcard on the day of travel: The issuing of the 'excess' is likely to incur a £10 'admin fee' * and the replacement will be sold at the price-tiers which are available at the time of issuing the 'excess', which may be higher than those which were bought at the time of the original booking. Furthermore, if the original ticket was booked on-line (and therefore attracted a VT East Coast on-line booking discount), that discount will NOT be available on the cost of the replacement ticket. Both of the above additional costs will frustrate any simple 'rule-of-thumb' calculation that it is only the removal of a discount of a third of the real price.
There might be an alternative solution which would involve seeking the guard's approval before boarding, and to ask for simply the difference in fares. But if the ticket held is an Advance, then a refusal will of course invalidate the ticket at a stroke.
* I do not agree that VT East Coast's application of a £10 fee for the removal of a Railcard discount should incur the "admin fee of up to £10" which is defined as applying to "a change of journey plans". But that still seems to be their practice.