When should I cancel my account with my previous web host ?

After the propagation period has completed you should contact your previous web host and request them to update their DNS and delete your domain from their servers. Once this has been done and their information has been completely propagated the transfer is complete. It is recommended to wait approximately two weeks after submitting for a site transfer to request cancellation of the previous account. This allows time for the few ISPs that take longer to update their tables to get up to speed and helps prevent losing visitors that are using these slower ISPs.