I've had similar and it took me a while to get my head around this. I think the presentation on the bill confuses things as they refer to the old speed charge and the new speed charge when I think in reality it is nothing at all to do with the free speed upgrade.
We are billed for the month ahead in advance ... in my case my September bill was in respect of the period 10th October to 9th November and was all charged at the old pre-price increase rate. But the prices actually went up on the 1st November so Virgin are simply belatedly recovering the difference between the old price and the new price for the few days 1st November to 9th November. Your dates will no doubt be different as we are not all billed on the same date, but the principle remains the same.
If that made any sense I can probably get away with another whisky.