Migrating will cause 3rd parthy launchers to fail!
Taken directly from the Technic Disocrd
If you’ve read the recent MC account migration (www.minecraft.net/en-us/article/java-edition-moving-house) and you use Technic Launcher (or any other custom launchers), then do not migrate your account (to a Microsoft account) or you will be unable to use the launcher.
The new auth system that Minecraft uses after this migration isn’t support by any MC launchers other than the vanilla launcher, nor do we currently have any information about it so we can implement it.
Do not migrate your account right now or you will be unable to use our (and other custom) launcher(s).
When? According to Mojang, you’ll be forced to migrate your account starting in early 2021. We hope to have support for their new auth system by then.
Apologies for the ping, but this is rather important.