I don’t think subjects should be changed today, because those use the threading in Outlook, the thread is now ruined. However people do, so I wish Outlook gave the functionality to change a subject and then use “Tools | Refresh Conversation ID” to change the Conversation ID.
PS: Better still just refresh the Conversation ID after you change a subject

Figure: Need an option “Tools | Refresh Conversation ID” for use after you change a subject