What do yo mean by end date with old provider?
to move ISP you should do the following:
check to see that your old ISP contract is out of the minimum term ( sometimes called out of contract)
you can try and haggle with your old isp for a better deal, if they dont match the competion ie Now broadband han up and do no more, do not cancel or say you will be leaving/cancelling
Sign up with Now, you will be given a go live date.
You will get a message from your old ISP saying something like sorry to hear you are leaving and possibly offer a deal, if you do nothing your internet /phone will switch without a hitch.
if there is a delay then your old ISP will / should continue to provide a service until Now take over and you will be charged the standard rate of your old ISP until switch over.
If you contact your old ISP and say you are cancelling and then they give you a date the service will end, this wil cause all sorts of issues with moving to, or setting up with a new ISP