Even though customer makes long term contract, such as Reserved Instances, before October 1, 2019, AWS will apply tax rate fee based on the time the services rendered. For example, if you purchased a one-year partial upfront Reserved Instance on August 1, 2019, AWS will not charge additional tax to Upfront fee that you already paid before. Regarding discounted hourly rate fee which will be charged every month until the contract ends, new 10% tax rate will be applied from October 1, 2019.