So, early next year, Mojang will start instructing players on how to migrate their accounts from Mojang to Microsoft. They emphasise in their post that this is mandatory, and if you don’t make the transition, you won’t be able to log in and play at all. It sounds kinda harsh, but it does seem to be for good reasons. Mojang immediately bust out the positives in reassurance, noting that Microsoft accounts will allow you to enable two-factor authentication, have access to better parental controls, and use chat and invitation blocking.
The account migration will mean all your Minecraft PC games (so Dungeons and Bedrock, if you own them) will be linked under the same account, but importantly, they won’t be merging. Java will still be its own thing: you’ll still have your in-game Java username, mods, skins and the ability to play with your Java pals. Any new players trying to sign up for Java will make Microsoft accounts rather than Mojang accounts starting this Autumn (so soon, if not now). Existing players will receive emails and help from Mojang at the beginning of 2021 to start making the switch. When you’re done, you’ll get given a cape for your troubles, too (to clarify: an in-game cape, though I think a real-life one would be much more fun).