Yes sorry, every major number release of Kontakt must be purchased as an upgrade. So going from 5 > 6 is a paid update, but going 6 > 6.1 is not. You will still get the too old message on sub-versions I believe however, just not the DEMO mode one.
I think you were meaning the plugin, not the Kontakt sound library? The K6 Factory Library has the same content selection as K5 and you should be able to load it into either version at its latest update, they didn’t do anything to it during that transition from K5 to K6 except for bug fixes / file compression (as verified by EvilDragon here: https://vi-control.net/community/threads/kontakt-6-factory-library-vs-kontakt-5.86403/)
Thanks for updating us though, I’m sure many were not aware that this was the case. NI seems to let people know this well through their sales but not much else