Cancelling/changing direct debit

Hi,

I'm wanting to change my bank account details for my direct debit and was told to pay the outstanding balance for my first bill before I could do so- which I did. However, before changing the details I saw the confirmation email states "Card payments do not reduce your Direct Debit amount. Your scheduled payment will be attempted as previously advised.", which seems to say that I'll be charged the payment for the past billing period again? Can someone please confirm whether or not I would be charged double the amount that I should be?

If so, I would like to cancel my direct debit altogether and instead pay manually each month, as it states I can cancel at any time. However, when I click Cancel Direct Debit the link to calling a staff member is not working- additionally, I have seen in a few similar posts that doing so damages credit score and/or incurs extra charges, none of which is disclosed anywhere on the site so I would like to know whether this is true or not.