Hi @Kathi ,
To upgrade from Reporting Services 2016 and older versions to Reporting Services 2017 and later, follow the article.
1)When you upgrade a Reporting Services installation to a SQL Server 2016 Reporting Services or later (SSRS) installation, existing reports and snapshots that have been published to a report server are compiled and automatically upgraded to the new schema the first time they are processed. If a report cannot be automatically upgraded, the report is processed using the backward-compatibility mode. The report definition remains in the original schema.
2)Yes there will be a down-time that the users couldn't access the current reports.
3)We'd suggest that just run one version you need.
4)SSDT is backwards compatible, so you can always use the newest SSDT to design and deploy databases, models, reports, and packages that run on older versions of SQL Server. You may upgrade your Visual Studio software as well.
previous-releases-of-sql-server-data-tools-ssdt-and-ssdt-bi
5)Here is an official article about upgrade-and-migrate-reporting-services for your reference. Please upgrade on your test environment and then implement it in the product environment. The most important thing is that back up the files and data.
There are also some third-party teaching blogs online, you could also refer them carefully.
Also you may create a ticket to Microsoft for Engineer to help if you paid for Microsoft products.
Regards,
Zoe
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