Thanks Julian.
I was not too complicated but not having any data model made the process longer.
I still have faced an issue with my converted users not been able to view the categories. I have manually addressed the issue by creating a "legacy" group, add the necessary group/priv to my categories and manually adding my users to the legacy group (fortunatly was having no more than 150 legacy users).
I presume it has something to do with group association but haven't been able to figure out what exactly