There are two cases we need to pay attention to here:
1. If you are on the Free plan and have never upgraded to the Premium plan, you only need to upgrade to the Premium plan to experience the 7-day trial. The app fee will only begin the billing cycle after the 7-day trial period. For example, If today is November 6 and you upgrade to the Premium plan, your billing cycle would start on November 13. See the image below for more detail:

2. If you have downgraded from the Premium plan to the Free plan before, the 7-day trial will not be applied if you continue to upgrade to the Premium plan at the moment.