RTC Upgrade. Custom plugins
2 answers
In addition to what Ralph said, keep in mind that RTC/EWM is n-1 compatible. This means a 5.0.2 client can still connect to a 6.0 server. So if you are referring to custom 'client-side' plugins, then as long as they were using official API, there would be no issue upgrading the server, and the client-side plugins should still work as normal.