Each monthly billing cycle starts on the day you subscribe. If you start with Tier 1 on February 1 and upgrade to Tier 2 on February 15, you will be charged the difference ($121) for the upgrade. The additional 3000 credits will be added to your account and available until the end of the current billing cycle, which is March 1. This way, you get the full benefit of the Tier 2 package for the remainder of the month without paying the total amount for Tier 2 when upgrading mid-cycle.