If you try to change your discord username (not server nickname), and someone else with your tag ID already has that name, your DiscordTag#0000 will change. People grow attached to their tag IDs, they become recognized by their tag IDs. There should be a warning for this that says something like: "Watch Out! Someone with your tag ID is already using this name. If you change your name to this, you will recieve a new random DiscordTag#0000. Are you okay with this? [Yes] [No]"