offers all of us the ability to select a different company (from BT) through which to route calls.
Normally, CPS has been used for long distance calls only, because local calls are cheaper - or no more expensive - via BT. You'll still receive bills from BT for line rental, but even that is set to change later in the year.
As far as I know you'll still be abled to dial into the Freeserve servers, after all, your line type or quality will not change. If there are problems you could still use the BT service for your dial up connection because you can override the Preselection at any time. Why don't you phone the Freeserve technical helpline and ask them to confirm that all will be well?