You might be able to make the changes by signing into My Virgin Media , however if you have an older discontinued package, or a discount on your package then you'll probably see the "Oops somethings gone wrong error". That means the account bwill need manual adjustments making and so you'll have to call in or do it via text.
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 a 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