Ring 150 from a VM phone or 0345 454 1111 from any phone -> Option 1 -> Confirm your details -> Option 1 (Billing) -> Go with the misc billing option. Make the request. Or make the request via the Webchat from a desktop or laptop between 8AM to 8PM M-F and 9AM to 5PM Sa. Select the Account and Billing -> then Understanding my Broadband, TV and Home phone bill -> Chat online. If the agents are busy, reload the page every 1 minute or so and check again.
Last I read you'll be given up to 8 possible dates to change it to. Any change will mean your next bill will be somewhat different from your normal bill, but after that it will normalize out.
______ Any opinions expressed by myself are entirely my own and do not represent Virgin Media in any way.