In 2020, developers announced a major update to Minecraft: Java Edition. Players will soon have to switch from a Mojang account into a Microsoft account to continue playing. That's 'soon' in terms of relative time, since we're not yet at the deadline however, the developers have just confirmed the date and sent out the final request for account migration.
You won't have the ability to play Minecraft: Java Edition if you have an account with Mojang account. The devs want everyone to switch to Microsoft accounts for "improved security and player security", and they're enticing people to make the change with the promise of free Minecraft capes.
Here are all the information about how you can migrate your account. Java Edition will be supported as usual following the change and will remain an independent entity from Bedrock. This is simply a reunification of the account system. Minecraft servers list 'll be able to keep your old username, too but you'll need to have an Xbox gamertag to match it.