Java Edition Locks Out Old Accounts In March
In 2020, the developers announced an important change to Minecraft: Java Edition, in which players would soon have to switch from the Mojang account to a Microsoft account in order to keep playing. This is a bit 'early' in a relative sense, considering that we're not yet at the deadline but the developers have just confirmed the date and put out the final request for account migration.
You will no longer be able to play Minecraft: Java Edition if you have a Mojang account. For "improved security, player safety" the developers are encouraging everyone to switch to Microsoft accounts. They are offering free Minecraft caps to encourage people to do so.
You can get all the information about how to move your account here. Java Edition will be supported as usual following the change and will remain a separate entity from Bedrock. This is a simple unification of the account systems. You can still use your previous username - all you need is an Xbox gamertag.