"3" contract phone? How do I change price plan?

[TashaM]

I have a £20 texter plan at the minute, but this does not have enough minutes for me, can I change to the £25 texter plan (which has 500 any time minutes instead of 200) midway through my contract? Or is the £25 texter plan considered mix and match, (then I wouldn't be able to change).

I called "3" and checked out the website, both useless when it came to helping an existing customer...