A 'regrade' is where they change the connection speed plan that you are on.
I think it is different from your billing plan.
Also please bear in mind that if Virgin are like Pipex they will renew your 12 month contract on the date of the regrade therefore locking you in again for a further 12 months.
This was something that I found out after my regrade as they 'failed' to make me aware of this fact.