Update (April): The date given by this file has since passed without any migration announcements being made, so account migration is still yet to come.
This migration involves moving users from their original Mojang accounts to Microsoft accounts. These accounts are used for logging in to the game's launcher to be able to play the full version of Minecraft.
The reasoning behind the account move is so that people can use the same account for all editions of the game, to allow for parental controls to be synced across platforms, and to add better login security measures.
New users who purchased Minecraft Java Edition recently have had their accounts automatically created as Microsoft accounts, so this migration is just for moving older users onto the new account system.
The new details about the account migration come from an interesting source.
A file titled
accountMigration.json posted to Mojang's launcher assets site apparently reveals important dates in the account migration process.
Three dates are listed on the file: the start date for the account migration and the start and end dates for when migrating your account will be forced on you.
Account migration for Java Edition users begins on 12 April of this year, according to this file. Starting from that date you will be able to transfer your Mojang account over to a Microsoft account.
From this date, users will have a just over a year to migrate their accounts in their own time, before they will be forced to.
Forced migration over to Microsoft accounts, according to this file, will begin on 12 May 2022 and complete on 5 July 2022.
When the voluntary migration period ends, says Mojang,
you’ll need to move your account or risk having it closed.
Note that Mojang has not yet made an official announcement about the exact details of the account migration process, so these dates may be subject to change.