Generally, no. Discussion threads begin with an original post presenting some sort of topic. There are no other permissive topics, especially if it's someone other than the thread-maker shifting the topic. Should the thread-maker, themselves, desire to change topic, they should just make a different thread. Naturally there are limits, so threads with more general/vague purposes have more flexibility. Still, if the thread is: "Naruto vs Sasuke", there should never be posts discussing Madara vs Sasuke, etc. Again, it can go either way sometimes, but in most instances the answer is no.