There is nothing in the retail guidance that says you can't change the class of travel. As long as, as you rightly say, the origin, destination and TOC▸ /route stays the same, then I don't see there being a problem.
^10 admin fee plus any difference in the price. A negative difference is not refunded.
So in my case I have a ^22.80 ticket - the cheapest advance on the train I now want is around ^15 - so I assume I'd just pay the ^10.