Change of route includes the facility to change ticket type, this is not uncommon, and it's in the instructions, that's what it means by to the "appropriate" fare, or "cheapest available ticket".
The Manual said:
Change of Route
National Rail Conditions of Carriage, Condition 13, permits customers to travel by a different route, from the one stated on the ticket or allowed by conditions of the National Routeing Guide, to make their journey on payment of an Excess fare. The Excess fare is calculated as follows:
Single tickets
The difference between the price already paid and price of the cheapest Single ticket, available for immediate travel, that allows the customer to travel on the route and Train Company of their choice
Return tickets - change of route in both directions
The difference between the price already paid and price of the cheapest Return ticket, available for immediate travel, that allows the customer to travel on the route and Train Company of their choice.
Return tickets - change of route in one direction only
Half the difference between the price already paid and price of the cheapest Return ticket, available for immediate travel that allows the customer to travel on the route and Train Company of their choice.
This example from The Manual proves that the ticket type can change with a change of route excess, unfortunately it is for a single, but the instructions above clearly state that for returns it's half the difference for one portion.
The Manual said:
Example
A customer holds an Anytime Single from Hereford to Alton routed “not via London”. The customer wishes to travel via London on a day at a time when a Super Off-Peak Single is valid. If the customer does not wish to break their journey, they should be charged the difference between the fare already paid: Anytime Single (£38.50p) and the Super Off-Peak Single (£43). The Excess fare is £5·50. Note: This example is not necessarily the current fare.
It's a MYTH that the ticket type must be the same, indeed the ticket type can be whatever is appropriate.
Indeed, given how each TOC seems to do it's own thing, and have their own set of restrictions and ticket types for any given flow, it's quite possibly the case that more often than not the ticket type will change. This is co-incidental and has no bearing whatsoever on the fact that the excess - for change of route - is 50% of the difference.
Further proof of this is in the East Coast Revenue Manual, that was leaked in an FOI request.
C7. How to calculate on-board Excess Fares
C7.1.1. Travel by a Different Route - with a higher fare
Return Tickets - Travel in one direction
Charge half the difference between the fare paid
and the appropriate Return fare for the journey
being made.
Railcard discounts allowed provided original ticket is discounted.
The excess is the same whether bought on the train, or at the ticket office.
In the specific case of the OP, what the retailer needs to do is select that it is a change of route excess, that a RETURN ticket is held (not single!) and that you wish to excess the return portion only. Fare paid is entered (£23.30) then the
appropriate ticket type is selected, SSR which is £30.35. The system should calculate that half the difference is then payable, that is
£3.55 (or £3.50 if rounding down)
