There is a specific way to change the SSRS service account if you strait forward change the service account then after restart of SSRS when you try to access reports you will receive below error.

The report server cannot decrypt the symmetric key used to access sensitive or encrypted data in a report server database. You must either restore a backup key or delete all encrypted content and then restart the service. Check the documentation for more information. (rsReportServerDisabled) Get Online

In the SSRS log you can see the below log

Cause:

The Report Server service uses the symmetric key to access the encrypted data in a report server database. This symmetric key is encrypted by using an asymmetric public key that corresponds to the computer and the user account that is used to run the Report Server service. When you change the user account that is used to run…