On NewNet's knowledge base (reference: articleid=541) they state
If your local exchange has been deployed to 21CN, NewNet will migrate you to the new service from February 2010 onwards, completely free of charge. If your exchange is yet to be upgraded by BT, you will continue to receive an up to 8Mb service on BT's 20CN. As soon as your exchange is upgraded, we will migrate you to 21CN free of charge
Nothing appeared to happen so I called the support line only to be told that migration would cost £11 (presumably plus VAT). Obviously I queried this only to be told that the knowledgebase was out of date and should have been removed.
One of the past benefits of NewNet was customer service - they are not particularly cheap. That benefit has gone so I think I will be moving elsewhere!