I always think it's worth checking whether you really need to change your monthly payments. Your direct debit has been calculated on the basis of your previous usage, so it should be roughly in the right area after a year, unless the weather or your usage is very different.
And Pure Planet check to see if it needs to change every six months. You should roughly be in credit each statement by an amount equivalent to your direct debit, because you are paying in advance.
So if you get significantly in to debit, I fully understand you may not want to wait to see how things pan out and/or Pure Planet's six month check. In which case follow Jon1's excellent advice...