Last checked: October, 19, 2023

In the migration guide there is:

Please keep in mind that your Microsoft Gamertag and your Minecraft username are independent of each other, and additional information can be found here. Your Minecraft username will transfer over automatically.

But the page doesn't exist.

(https://help.minecraft.net/hc/en-us/articles/360052640612-The-Differences-Between-Gamertag-vs-Username)