When you change your display name it sends a message like:
"russell has changed their display name to Russell"

When a room has a message from you it shows up as having new messages.  So
changing display name globally puts a new message in every room and obscures
which rooms you have read all the messages in which is annoying.

Also I think that there should be an option to have a message sent from my
account on another device to count as having read all messages in the room.