Contract changes

[KingAmo]

When I bought my phone in-store I chose a £25 contract plan as that was the minimum available. But then since I wasn't using the data and text I decided to go on Pay As you Go. But when I went to the store they said they can't do it at that time so they will first switch my data plan to £14 monthly then I should come again 3 days later to switch to PAYG.
Two days later I contacted the customer service through the O2 App and they changed my plan to PAYG. I was advised I will be charged for my monthly bill......which was then £14+ extra charges+£0.38 for earlier cancellation of contract.

But just now when I checked my bank account I saw that £25 have been deducted even though I haven't received my final bill yet.

Can anyone help me?
Why did they deduct £25
Will they refund me incase?
How can I contact O2 for help on this issue

[MI5 · Accepted solution]

@KingAmo 

You pay a month in advance so it will be refunded when the contract change goes through the system.

[KingAmo]

So when my final bill comes they will deduct that one and refund me any excess right?

[MI5]

@KingAmo 

Yes, correct