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I'm pretty sure they won't mind you offering to give them more money but you never know. Why not simply ring up and ask the question?
Why on earth would you pluck for the £30 tarrif? You only get 75 minutes!
 
Yes - i'd imagine if you rung customer services they could sort the change within a few minutes.
 
They certainly will. I asked in the store because I was concerned the 30 plan just might not cut it for me.
 
I've called them up before because I wanted to move down (was going to Germany for a month. Roaming sucks).

I got exactly the same information as the first reply: you can move to a more expensive (or equally priced non-iPhone plan) at any time, but you can only move to a cheaper plan after 9 months.
 
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