Home loan

In December 2015 I called to have my loan debited early on the 15th instead of the 25th because I got paid early.Which was no problem at all to change.The lady I spoke to advised me to call before 15 January to reinstate the usual debit date which is 25th to avoid being debited on the 15th again.Today I called Home loan section to change the date back to 25th.I was told I have to write a letter stating that and I have to send it through after the 18th.First of all it doesn't make sense why ahould I submit a letter after the 15th when my account would have been debited already.2.In December I was not told about writing a letter but I was advised to call and the date would be changed without hassle.I even told the lady I was talking to that I do not have access to the emails/fax at the moment until after 15th which really didn't bother her at all.She kept on telling what I must do,what she is telling me to do.I dont understand why is it an issue to change the date on the system.The most annoying part is I was never told about writing letters otherwise I wouldn't have paid early in December. She did not care about what I was telling her even when I told her how unhappy I am