It's long, but it needs paragraphs.
O2 will attempt to upgrade, not cancel the contract.
I can't tell what will happen in Exeter, but, most likely, there won't be much stock, thus your mother will need to be quick to get there. The process shouldn't last longer than 30 mins, and total time depends on queue and stock.
I doubt there will be stock at 1620, but it might be worth for her to go and have a look, or call after the conference.
This is why, I preordered online.
I will say one thing though. If anybody at the Exeter store has "reserved" a phone, if they don't turn up for it then it gets release to be re-bought, but that's not till the end of the day. So if i was you, id pop down like 20minutes before store closes, ask if any are "reserved" but so far uncollected, if they say yes, (which i expect some will be at every store everywhere) then wait the 20mins and pick it up