If I decided to remove K2 what will happen with those materials which were constructed with K2? The site admin didn't use any additional functions - it was just an experiment. May we return to "pure" joomla and save k2-materials?

Hi.

That is not possible.  K2 handles all the content itself pretty much bypassing Joomla so it does not use the Joomla articles - even if you do not use any of the advanced features.

I would suggest you use a K2 to CSV export plugin and reimport into Joomla standard acticles.

Export using: goo.gl/vJSBf

Import using something like: goo.gl/I7R5q

Good luck.

--Olivier