Back in May, Minecraft announced that the Mojang or Java versions of the popular game would no longer be valid, and you'd have to connect the game with a Microsoft account. Many have already done this, but the deadline is approaching.
The 19th of September is when your Mojang account will no longer be transferable to a Microsoft account, meaning if you've got a paid version of Minecraft, you'll be locked from playing it. This isn't going to affect a majority of people, but it can suck to lose out on a game you've paid for.
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To migrate your account, you can check the announcements on your Minecraft Launcher, or find the webpage online. From there, it only takes a couple of minutes, and you can set up a new account or link to an existing Microsoft account.
Read more here.