Changing your Username in Roblox will cost you 1,000 Robux, equivalent to $14.98. Whereas, the display name feature debuted in 2021 which is different than changing your username.

Change Roblox username or Roblox display name When you initially sign up to play Roblox, you must give your Roblox account a Roblox username. Other players will see your Roblox name in game.