Hi Random,
You can downgrade speed at any time as long as you have had your current level of service for at least 30 days. Upgrades can be completed online from
www.ntlworld.com website, downgrades have to go through customer services 0800 0522000. Either way changes must be kept for 30 days before they can be changed again. Billing will be pro-rata if changed during your current billing period.
Hope this helps
Gary
Ooops Debsy has already beaten me to it!