Change of debit order date too complicated

When I app**** for my medical plan I clearly stated that the debit order date must be on the 25th of each month. Discovery proceeded to run the debit order on the 1st of the month instead. I requested that it should be changed to the 25th as requested, but the agent said in order to effect that I will be billed double on the new debit order date???? I cancelled the request, because that is insane.