I wonder if they'll let you 'upgrade' to PayG. Can't find much info about this on *any* carrier.. it's not a common scenario. OTOH doing so would save me a few quid over the Simplicity contract I'm on now (and no 18 month lockin, which is kinda important).
I very much doubt it, for the simple reason that O2 would probably not regard it as an upgrade.
The most you have to lose though is the 30 days notice period, isn't it?