Bill too high after renewal

[#1]

Please can someone assist me with my broadband bill following contract renewal. I understand that the first bill of a new contract can sometimes be higher and that this may balance out in next month's bill, however I can't afford to make a higher payment so need this month's bill to be reduced to the correct amount.

My new contract starts on 17th June, £27.50 a month for 12 months. My previous contract was £29.62 a month.

My bill for 16th June (due to be paid 24th June) lists the following:
£12.81 Total for 16th to 27th June
£35.22 Total for 28th June to 15 July

Any help with this would be much appreciated.

[#2]

You automatically get a prorata bill when upgrading just before your next bill as it take a few days to update on the billing system. 

You will get another prorata bill next month and there will be a credit back for the over payment made this month at the new contract rate. Does that make sense?