I have had a contract for the past 24 months with ID mobile costing £42.66 a month. My direct debit would come out on the 20th of every month. On the 16th of this month, I upgraded my contract to a new plan costing only £7 a month. However when the 20th came around, I was still billed £42.66 through my direct debit. With the pro-rata explanation on my billing page, I was not expecting to be billed at all this month (as I was roughly £24 in credit). 
Do I need to cancel the old direct debit and create a new one for my upgrade? If so, how do I go about getting a refund for this?