The biggest changes to make to existing codexes are
1) Delete BRB rules/profiles from codex file - they are now all present in GST and able to be linked to.
2) Move any faction specific detachment/force type (Great Waaaaaagh, Angelis Strike Force, etc) into the codex file, then delete from the GST. - This will ultimately serve to ehance user experience by eliminating useless force types. In BS2.0 you select your army first, then your force. So you choose the army, and then you'll only see the basic detachment choices + codex specific ones, instead of everything under the sun.