So have existing 2016 SQL instance and SSRS on same box. They need to upgrade ssrs to 2019. I was going to do in place for sql instance first (it has reporting sever db's) and a few others. For the SSRS upgrade do I need to uninstall SSRS (after backing up keys) and then do install of 2019 ssrs and point it to the upgraded sql instance?
If you are doing an in-place upgrade, the installation will pick up all of the components of SQL Server, and upgrade them. If you uninstall SSRS, you will not be upgrading the database. I suspect that there are changes to the SSRS databases that an upgrade will perform.