Every bill contains the end date of your current contract. If you have online billing you can check on your bill by signing into My Virgin Media - you can also check your contract from there.
You can negotiate a new contract at any time, however best deals are when it's coming up for renewal.
To recontract you could try the text messaging service. Just send a text with a description of regrade to 07533 051809. Your message will go into a queue and a representative will respond to you as soon as possible. It may take a few days.
You can also call 150 from your Virgin landline, or 0345 454 1111, options 1,1 and 4 (Changes to your package) butbest to call at 8am to avoid call queues
The Service you do for others is the rent you pay for your room here on Earth - Muhammad Ali