This description in the

add_tags()

doc intrigues me tho, I would like to remove a tag from a dataset and add it to another version (eg: a

used_in_last_training

tag) and this method seems to only add new tags.

I see. Then I think you would need to do this via the API:
https://clear.ml/docs/latest/docs/references/api/tasks#post-tasksupdate
or
https://clear.ml/docs/latest/docs/references/api/tasks#post-tasksupdate_batch