Have a read here and here for more info.
You can change your username at account.mojang.com/me ... just sign in using your email address and click 'Change' next to your username. Note that you can only change names once every 30 days and that your old username is 'held' for 37 days before it is released for use by anyone else. That way you have the chance to change back if you decide you prefer your old IGN.
If you have an old MineCraft account that you log into using your username only and not your email account, you will need to migrate it to a Mojang account before being able to change your name. You can do that via account.mojang.com/migrate.
Changing your username will not affect your plot ownership, plot permissions, guild membership or anything else on MineRealm. Everything is now referenced by your UUID in the code, not by your name. Bans are also referenced by UUID so the existing ban list remains current