hands on their face, pursing their lips, rolling their eyes, turning away, stop talking, looking away or at something else

False. Their face and voice should be neutral. If the persons face and voice are excited or upset, the current topic is still important to the person.

True. example: can link birthday presents (old) to favourite game (new)

you've 1-2 comments about the current topic and you want to talk about a new topic, the other person looks bored, or there's a pause in conversation

speaking of..., that reminds me..., by the way..., off topic, but..., oh, have you seen...