Just had a product change and been charged for exceeding usage!

[#1]

Morning,
Having fibre fitted this morning, still awaiting the engineer to turn up.
Package changed from 60g allowance to 40g because I'm changing from plusnet extra to value fibre. Received two emails at 825am saying I have gone over my usage allowance and am being charged an extra £10!? 
Eh? Surely my usage should zero again from the time of having my new service started, and is not continuous?
Can someone please confirm this. If this is the case then I'm not happy and would of waited until my next billing date to switch services.
Cheers

[#2]

Indeed.
This looks to have occurred because your usage month and billing it until the 13th and your account has changed earlier than this.
I have applied a £10 discount to credit this extra use off between not and the end of the 13th. This means you have just under 5GB available until then. If you use more, we will look to correct that too, upto the 60th GB per your prior account.

[#3]

Nice one.
Really appreciate it.