ALR would be subjected to an admin fee, which vary by TOC, from £5 to £20 AFAIK. If it is entirely unused, (because the ticket hasn't started yet), that would be the only deduction.
If it has been used, then an admin fee would apply, and they would probably divide the cost by 7 or 14, times that amount by the number of days you have used it then subtract that from the ticket price, subtract the admin fee and whatever is left is the refund.
I very much doubt that any ticket office will simply non-issue a ticket and re-issue it with new dates. That isn't permitted under most internal cash regulations. You may have to buy a new one and send the original in BEFORE the ticket has a chance to become valid.
I would imagine an admin fee would just be subtracted from the sleeper supplement and whats left is yours. ALTHOUGH the "rules around refunding supplements etc aren't exactly clear, (if they even exist in the first place).