I should have been clearer in my previous post.
I agree that, where "void" days were added when the ticket was renewed, they should be deducted prior to calculating the base rate for that ticket for change-over purposes, then added back on when issuing the new ticket.
However, if there are any that have become due since, iKB (and The Manual before it) states that they must be refunded prior to doing the change-over calculation.
Picking an example at random:
Passenger holds a season ticket from Alton to Waterloo (SWT Mainline Group 4). On renewing the ticket on 1st January 2014, they had 5 "void" days added, extending the validity of the new ticket to 5th January 2015.
They then apply for a change-over to Aldershot to Waterloo (thus remaining in the same charter service group) on 20th August 2014.
When calculating the base rate, the 5 additional days are deducted, and the relevant fares divided by 365 as normal.
Since they renewed their ticket, two additional half days have been accrued (8th and 10th July). A refund
must be processed for these prior to doing the change-over. The additional day
can not be added to the new ticket (extending it to 6th January 2015).
As for charter discounts, it's mostly common sense that a passenger shouldn't get "credit" for poor service they haven't experienced, and nor should a TOC be "penalised" for poor service they haven't caused.
Some further examples:
- Customer holds a Slough - London Terminals season with a 5% charter discount, wishes to change to West Drayton - London Terminals. The discount can be transferred, as both journeys are within the same charter service group.
- Customer holds a Slough - London Terminals season with a 5% charter discount, wishes to change to Gerrards Cross - London Terminals. The discount can not be transferred, as no discount applies to the new journey.
- Customer holds a Gerrards Cross - London Terminals season with no charter discount, wishes to change to Slough - London Terminals, where a 5% discount would apply. The discount can not be applied to the new ticket, as the customer hasn't been travelling with FGW.
Having said all that, it's unlikely that the OP would be able to do a change-over where less than a month's validity remains on the original ticket, due to the limitations imposed by some Ticket Issuing Systems, which still apply the old rules.