How does it work when a tree gets updated?
For example : lets say i finished the korean tree when it comes out and it gets updated into tree 2. Will the new content just be at the bottom of the tree or does it work in a different way?
Depends. Smaller updates may add just a couple of lessons to skills you have already finished and maybe the occasional new skill in the middle of the tree. Team Korean however has said that they are cutting the tree in half for an earlier release, so you may see a lot of new skills added to the end of tree.
If you already finished the tree, they take the trophy away. Just either a lot or a few lessons are added to each topic, or new topics are created.