Direct debit issues

I moved house at the end of May and kept the property account with Bulb as no other supplier was taking on new customers. I set up a direct debit but the amount Bulb chose to take was over twice what I use, so I tried to change the amount online - sadly you can only increase the amount you pay, not reduce it.
I then asked to go on the pay your bill when it arrives scheme, and was instructed to cancel my direct debit. I did that, and am now getting emails saying I need to set up a DD and may incur charges if a payment is missed. I only found out that you shouldn’t cancel a DD by looking on this chat line!
I’ve built up enough credit in the two months I’ve been with Bulb to pay the next bill anyway, which is why I want to change. So I wonder:
a) why was I told to cancel my direct debit;
b) why I’m being threatened with charges when I’ve got over £100 credit on the account;
c) why I have to find out important details (like don’t cancel your DD) on a chat line.
Not hugely impressive for my first experience with a new energy supplier! Could you maybe put something on your website to explain why you shouldn’t cancel a DD, and make it possible for people to reduce the amount taken from them if they’re always in credit?
I realise that Bulb need money but not giving customers the full facts or control of their accounts is not the way to build up trust!