If you're looking to upgrade to SQL 2016 or SQL 2017 and you use reporting services with custom authentication - then FYI it will fail.
You will have to find a way to rewrite your custom authentication DLL, see my MS forum post - I'm not the only one, https://social.msdn.microsoft.com/Forums/sqlserver/en-US/f474ba23-7e8d-4c6b-ad41-b2327956226b/sql-2016-reporting-services-custom-security-how-to-access-name-of-item-being-accessed?forum=sqlreportingservices

Also you can only install 1 instance of SSRS 2017 per machine - uber-sigh. MS give with one hand (SQL 2016 SP1) but take away with the other!