'On a different note' is a phrase people use when they want to tell the person they are talking to that they want to change the topic of the conversation. In public speaking this is called signposting and helps to prepare the audience for the change and avoid confusion. Other similar expressions are 'Now what about...' 'My next question/problem/ point is..' We also say 'on a lighter note' when we want to talk about something less serious.

It means to change the subject to something different, usually when speaking to someone.