How to EDIT posts Terms?

[rdzak]

I'm trying to EDIT (remove, add new etc) Terms in post.
Currently I'm getting a post and editing its other properties (title, content) by assigning new values, however that doesn't work for LIST of terms. I'm ale to create new post using terms_names, which is fine but how to edit existing one? I have tried using post.terms property and assign new list of Terms (instances of WordPressTerm), which I retrieved using filter (or simply id):
new_ terms = []
new_terms.append(client.call(taxonomies.GetTerms(taxonomy, {'search': term_name})))
And now assign it to existing post:
existing_post.terms = new_terms

That doesn't work as even though I'm posting it, no new values are showing.
Can anyone help?