Use Report Server Configuration Manager on source server, click on Encryption keys and backup. You’ll need these on the destination server
use Report Server Configuration Manager and stop the service on both servers
detach ReportServer and ReportServerTempDb on both servers
I advise on the Destination Server making a backup of these two database
Copy the ReportServer and ReportServerTempDb and the encryption key file to the new server and attach them in Sql Server management studio
Start the service
Click on Encryption Keys, restore and restore the key file generated on the source server.
Subscription users are going to be different, you have to update the users to the new server. They’re in the format of ServerName\username
select * from subscriptions in ReportServer database. Run a simple update query.