Renaming a member?

[Hall of Famer]

There are also columns from four tables that you may need to change too:

table prefix_folder_messages: two columns(`fromuser`, `touser`)
table prefix_pounds: three columns(`firstowner`, `lastowner`, `currentowner`)
table prefix_promocodes: one column(`user`) 
table prefix_shoutbox: one column(`user`)

Anyway changing username is an extremely tedious process and Id definitely not recommend you to do this unless you have a good reason. And just like Kyttias mentioned, you also need to keep track of your username from your forum database if you have forum integration, which I cannot help you since I know little about MyBB's table structure other than its users table.