I know I've just posted this on another thread but:
When the 20Mb upgrades were being done they had a page that detailed the upgrade schedule and also said 'Do you need a new modem?'
HERE (from the wayback archive) if no-one believes me
If you did need a new one, you could then order one for free through the same site
So they DID tell people and they DID offer a way to get it replaced. If a customer then hasn't bothered, it is hardly Virgin's fault