Starting September 19, 2023, at 11:00 am EST/17:00 pm CEST, unmigrated Mojang accounts will not be able to sign in to Minecraft.net or the Minecraft Launcher to migrate. We’re doing this to ensure that everyone is playing using accounts with improved security and player safety.
Source: Minecraft.net article

What will happen to unmigrated accounts?

Once the deadline for migration is reached, players will no longer be able to access unmigrated accounts. All unmigrated accounts will be deleted at a future date.
Source: help.minecraft.net

What if you don't have access to the email address associated with your Mojang account, or have any other queries?

The details are unclear. The Minecraft help center link says "Minecraft player support will no longer be able to help with migration-related issues after September 5, 2023." whereas the minecraft.net article states "[...] starting September 19, 2023, Minecraft Support will no longer be able to assist in any MSA migration-related tickets."

What do you get for migrating to a Microsoft account?

Microsoft claims improved account security, and 2FA. You will get a Migrator cape and access to Minecraft for Windows (Bedrock Edition) on the Microsoft account chosen at the time of migration.

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[–] Pxtl@lemmy.ca 13 points 1 year ago* (last edited 1 year ago)

The players still have their stuff and their user ID, just under a new login process. They've been pestering users to make this migration for years and years.

Edit: also, this is Java edition, meaning the worlds they built are just Minecraft save files that a new user could access. The cloud-based one was Bedrock Edition, that's the one where you'd have cloud-based worlds that you could lose if you lost your account.