I'm new to K2, have just installed it, and I'd like to know how to change my existing previous articles, which have been posted in the "quick icons camp" categories, to the new K2 categories I have created. Do I have a mass transfer system to do it?

There is an import articles to K2 button. You can use this button.
These videos will help you getting started: getk2.org/documentation/k2class2012