I'm more interested in the "time" excess; upgrading from off-peak to anytime, or similar. I presume that's a fairly simple calculation? But it would have to be done before boarding, at stations with ticket facilities?
It's a calculation that the staff find very difficult to do ... I had one poor chap try three or four times until he came up with a price that sounded about right, and another with an
Avantix▸ on the bridge at Paddington pass me to his colleague because he wasn't sure how to do it.
Beware off peak to anytime on your return if you're on a period return - it may be cheaper if you're making a later journey to buy a peak single, and use the return part at a later date if it's a regular journey. This caution applies to
GWR▸ flows but not so much to Crosscountry flows, with their differing differential between single and return (need to think that one through though)
Note you can also excess a route. Return, Westbury to Swindon via Melksham can be excessed prior to return to "any route" allowing travel via Bath if you miss your intended return run of the "Melksham Flyer" and don't want to wait over 2 hours for the next one!
If you can't get it done at the station, ask the train manager as you board if he can come round and do it for you.