Hi, I did the same. It was explained to me I have to pay the remainder of my original plan plus my new unlimited plan which worked out £7. I glad I didn't have to start a new contract as some networks do that. 

How much more was your bill? 

Hi ​@Raihan Ahmed,

I've just sent you a private message to help get this sorted.

To find it, click your profile picture in the top-right corner and select ‘Private Messages’.

You can also use this direct link to your inbox: https://community.idmobile.co.uk/inbox/overview

I'll speak to you there.

Thanks.