Hello hanspumpel
You can safely upgrade RosarioSIS core. You won't lose the Premium add-ons.
Regarding add-ons though, I cannot guarantee old Premium add-ons will work properly with the latest RosarioSIS version.
I invite you to check the WHATS_NEW.md file for notes inviting to upgrade add-ons for compatibility with a RosarioSIS version.
That being said, I am pretty sure old add-ons will work properly in your case: I can only trace back an add-on upgrade notice for version 5.9.