All I need done is a Minecraft Forge mod done (for 1.15.2), where once the game would load into the main menu there is a new GUI.
The GUI needs a text input with a "Continue" button, that once typed and the button is hit, the Clients username is changed. This would need to work for offline servers and singleplayer. After the username is hit they are brought back to the main menu like normal.
In the Main Menu, there simply needs to be a button that brings you back to the name changer.