How to change the login for a database's dbo user?
The group or login that is member of the sysadmin role is indepentent of who owns the database. The login that is mapped to the dbo of the database is basically the owner. You can change this via SSMS or with the query below:

ALTER AUTHORIZATION ON DATABASE::[ReportServer] TO [sa];
ALTER AUTHORIZATION ON DATABASE::[ReportServerTempDB] TO [sa];


You can find more information on the specifics about the database owner (dbo) from this question: What is the purpose of the database 'owner'?