Sometime in March you will be given the option of upgrading.
If you are on 300k at the minute then you will be given the chance to upgrade to 1mb at no cost, but your 30gb monthly download limit will drop to a rather poor 3gb.
750k will be upgraded to 2mb and 1.5mb upgraded to 3mb.
As far as I am aware NTL won't be actively advertising these changes, so mid march sometime I would advise that you phone NTL to query the changes.