MFC changing debit order date without authorisation.

How is MFC authorised to just change my debit order date to whichever date they want. I specified a date when I signed my contract. The date I specified was specified for a reason.

If they decide to just change the date without my consent, it can possibly result in a debit order rejecting and then it will be my credit record that is affected due to them making unauthorised changes.

Since inception of the contract I have not missed one premium, so I fail to understand why the sudden and unauthorised change is being implemented.