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, or because the package you are on is no longer sold and requires 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.
You can also call 150 from a Virgin landline, or 0345 454 1111, options 1,1 and 4 (Changes to your package) but best to call at 8am to avoid call queues, or late afternoon around teatime
Dave
The Service you do for others is the rent you pay for your room here on Earth - Muhammad Ali