If in the the upgrades and offers section you see an Oops something went wrong message, it's because you either already have a discount and the systems require an extra bit of manipulation by the agents to set up any new discounts, or because the package you are on is no longer sold and again requires the extra manipulation by agents to change the package.
If you don't want to be tied up on the phone then 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. And is a little more frustrating than phoning.
You can also call 150 from your Virgin landline, or 0345 454 1111, but best 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