Is there a deviation of route to the new destination? If not...
Over-distance excess on the return portion of an Off Peak Day Return is the difference between the fare paid and the appropriate Return fare for the throughout journey.
Or, if cheaper, the appropriate Single fare for the extra journey, provided the combination of tickets complies with National Rail Conditions of Carriage Condition 19. Effectively meaning, as neither one of the tickets will be a Season/Leisure Travel Pass, that the train you are on must call at the original return destination.
This should be bought prior to travel. Some ticket clerks won't issue an over-distance on a return portion after the outward journey has been made.
It would still help to know the origin and destination of the original ticket, and new return destination, for accurate advice.
Cheers BNM, I was thinking in general terms rather than a specific journey.
TBH▸ if I were to use this in future, I may just plumb for the anytime single because of what you've said regarding some ticket clerks not issuing if the outward portion has been used (depends on the price difference, I cannot be bothered to have arguments now for a few pence - leaves a sour taste in the mouth which is not worth it
IMO▸ ).