"Now just to be clear, migrating from Mojang to Microsoft accounts is mandatory. If you don’t make the move, in several months you won’t be able to log in anymore – which means you won’t be able to play either. But no need to worry, because we’ll give you detailed and clear instructions on how to do so. You’ll get the information you need through this site, an FAQ on our support page, videos on our YouTube channel, and even emails straight to your inbox."
Full blog post here: https://www.minecraft.net/en-us/article/java-edition-moving-house