Best answer by Transparent
Firstly, I think you should read what one of the Moderators said in answer to this other question about the relative dates of Statement and Direct Debits.
OVO follow Ofgem guidelines in setting these dates. In effect you pay the next month in advance rather than pay for the amount which is shown on your latest Statement.
Once a Direct Debit is set up, it's quite hard to move the date. I'm not in the finance sector, but I believe it involves ceasing the existing Mandate and creating a fresh one.
It's relatively easy for OVO to move the Statement date, but doing this can be detrimental. Other contributors to this Forum have complained that this practice leaves their accounts with OVO in credit to an amount almost equal to two-month's worth of Direct Debits.
The reason for this is that the Guidelines attempt to bring your (annual) contract to an end with your account showing a small positive balance. If the account shows a minor debt, then you will experience difficulties in switching to another supplier. So there are established practices for ensuring that you don't fall into this trap unwittingly!
Before you contact OVO directly, have a look also at this page which shows how Estimates and Direct Debits are calculated.
Please ask again here if you want further clarifications.