From https://help.minecraft.net/hc/en-us/articles/19615552270221
I MISSED MY CHANCE TO MIGRATE. WHAT NOW?
While you won’t be able to keep your skins, progress, or Java profile name, you can complete the following post-migration process before December 18, 2023 to experience Minecraft: Java & Bedrock Edition for PC with a Microsoft account:
1. Log-in with your Mojang account email address here.
2. Click on the 'verify Mojang account email address' button.
- Enter your Mojang account email address. Next, click ‘Get Code’ and a code will be sent to your Mojang email.
- Enter the code.
3. Enter a Microsoft account email address.
- This email address can be from a new or existing Microsoft account that you own.
- Click ‘Submit Request’. 4 Check the box next to ‘I understand that my Mojang account will be deactivated’.
- Double-check that the Microsoft account email displayed is correct.
4. Click ‘Finish’.
5. Receive an email confirming that we’ve granted your Microsoft account an entitlement to Minecraft: Java Edition and Minecraft: Bedrock Edition.
6. Download the free Minecraft Launcher through either Minecraft.net or the Microsoft Store. This article provides more details on how to download the Launcher.
7. Log into your Microsoft account through the Launcher and check that you have access to both Java and Bedrock.
My take:
Fresh UUID:
A second chance anywhere you had a username or UUID ban
Throws off stalkers
You lose your skin history if you don't back it up first from NameMC
You lose progress, ranks, Mojang capes, pets, and access to any containers or features hardcoded to your UUID. A server like Hypixel won't help you regain anything you lost.
Clean Name History:
Free prename
The name API is more lenient about requests sent by prename accounts
Another name day is distantly upon us
Circumvents Mojang's criteria for not removing certain names from your name history
If you sell accounts, accounts that had once had more than one name in their name history instantly become more valuable
If you liked your name, it will drop, but not for at least a few months
It's free for currently unmigrated Mojang->Microsoft accounts and there are no questions asked
Mojang won't offer support about this if you have problems, because it's related to migration, and migration is over
Unmigrated Minecraft->Mojang accounts cannot participate, only unmigrated Mojang->Microsoft accounts.
If I had known this was coming I would have held off migrating entirely! Praise Lyicx! 
