No you don't cancel, just phone them up and ask for a MAC code, which you should receive no longer than 5x days, the MAC code is valid for 30 days, so if you don't use it it just becomes invalid and your service just continues as normal with your provider.
If you use the MAC, then all you do is give it to your new provider and they sort it all out for you, they will give you your new details, username/password etc... for logging on, then in about 7x days time you will find that you can't log on to your old provider (which you are still on for the 7x days during the migration process, or for however long it takes) anymore, so all you have to do is put in your new details (username/password), and click connect, and thats it all done, your away on your new provider, the only down time you should have is how long it takes you to change your username/password over in your modem/router and click connect.
