Auto-renewal issues when changing plans

I've recently joined Smarty and paid for the £10 (60 GB) plan on 8th March and turned auto renewal payments off.   I have now decided that I would like to change to a cheaper plan but read you have to turn auto renewal back on to change it.  However when I turn auto renewal back on it's asking to take payment of £10 and sends a verification code to complete payment!  Why is this happening?  TIA