Got the same email, had the same on-line 'you can't change your package on-line' finally got to speak with some one, well 9 people and 11-12 hours on the phone over the last week, upgraded my package, its only available to Ultima Oomph contracts..
.I'm now being told that its not available in my area (07) so why send me the email. Virgin are very frustrating to deal with....fyi, I got one of the advisers I talked with to raise an issue due to 'not able to update on line' and they sorted it, so persevere