Welcome to Flyertalk Crisphoarder, welcome in particular to the BA forum. It's good of you to slip out of lurkerdom and join us, I hope this will be the first of many posts. I don't think you need to worry about nicking the odd bag of crisps, as far as I know it's nuts only in Club World. Welcome on board.

If you have a flexible ticket it is perfectly reasonable to trade it in for a cheaper ticket should it become available. The traditional reason, however, that people end up with these tickets is that inflexible and semi flexible tickets have gone off the market and destined not to return - either due to high sales or late booking. (The other reason, obviously, being the need to be flexible, including endorsement to other airlines, but in the world of e-tickets the latter point has been blown to the winds somewhat).

So that's about it really, if you buy a lower cabin, then you certainly can't be sure of an AUP to CW in this scenario. An AUP to First is always possible, but I suspect it may cost the same irrespective of your ticket cost. The one thing that may be cheaper is a full upgrade - not POUG or AUP - to restricted First, if that is available. You should see the price of that now if bought online, or you should work the cost out at least. If it is a travel agent ticket it is indeed best done at the airport.