We have Responsive HTML5 help source that is created by using RH2017. While all team members use RH2017, what if we have 1 team member on RH2019?
Would he/she be able to update the same source files of RH2017 seamlessly without any hassles?
Does the Responsive HTML5 output of RH2019 and RH2017 differ greatly? Any risks?
No definitive answers to this one.
A project created in 2019 Classic can be opened in 2017 and most things should work. However, there were some changes and going forward Adobe could apply an update to 2019 Classic that breaks compatibility. Even Adobe cannot guarantee that will not happen. It's not a scenario that I would risk.
If you are referring the new UI 2019 version that also ships, there is no compatibility. The code is very different.
It could be worth contacting Customer Services to see if they will sell you another copy of 2017.
See www.grainge.org for free RoboHelp and Authoring information.