Agency changing payment terms

[DigitalUser]

My agency has recently decided to move over to a 'new billing system' which has meant they want to delay payment (which used to be the week after the invoice submission date [usually the first week after the previous month of work]) to the end of the following month i.e. a delay of 3-4 weeks compared to previous payment arrangements. Apart from growing a pair and negotiating, how have other people dealt with this in the past?

[northernladuk]

This place must be sockie heaven nowadays. We get one question on a topic and within a day we get three or 4 on exactly the same topic...