The transition process from older Mojang accounts to newer, more secure Microsoft accounts began for Java players back in June of last year.
From last June until 10 March 2022, upgrading to a Microsoft account has been completely voluntary. From mid-March, all players will be forced to upgrade their Mojang accounts or else they will be locked out of playing Minecraft.
Mojang states that Microsoft accounts offer players more security measures to protect their accounts, such as two-factor authentication.
Although Mojang says the process is quick and simple, some users have reported been locked out of their accounts once migrated, with Microsoft's account recovery process seeming ineffective.
All players who migrate upgrade their Mojang account receive a "migrator's" cape that can be equipped to their player character.
More information about upgrading accounts can be found in the upgrading FAQ on the game's help website.