I wanted to do a debit order date change

I wanted to do a debit order date change. Currently I'm debited on the 25, I wanted to change to the last day of the month effective May. The agent told me that I'll still be debited on the 25 and the pro-rata will be debited on the last day of the month. Doesn't make any sense at all. Had absolutely no issues doing the changes with my other policies at other companies. I'm so bored and annoyed nxa