IUA Business Solutions' terrible customer service

IUA Business Solutions decided to change my debit order date to the 29th December 2023 even though they have been made fully aware that I only get paid on the last day of the month. I have tried to phone IUA Business Solutions but their call centre staff are very rude and tell me refuse to change it to the last day of the month. After 4 years with IUA why do they suddenly decide to change my debit order date without my permission and then refuse to change it back? I will not pay IUA on the 29th December, I will make sure to withdraw all of my funds before then and just to be on the safe side I will be informing my bank not to allow any debit orders before the end of the month. IUA cannot change the goal posts on a whim without my permission and think they can get away with it.