In 2020, developers announced a major update for Minecraft: Java Edition. Players will soon have to switch from a Mojang account to a Microsoft account to continue playing. While it's a long time away, the developers have confirmed the date and issued the final call for account migration.
You will no longer have the ability to play Minecraft: Java Edition if you have an account with Mojang account. Mega blog want everyone to switch to Microsoft accounts for "improved security and player safety" and are entice players to switch by offering free Minecraft capes.
You can get all the details on how to migrate your account here. Java Edition will continue to be supported as normal after the change and will continue be a separate entity to Bedrock. This is simply a reunification of the account system. You can keep your old username. All you need is an Xbox gamertag.