Hello Russell. Thanks for getting back. The fact that your former username is still available is a great indicator that your account is intact. The problem is probably that the Minecraft Launcher on you new computer isn't recognizing the Microsoft account that owns your license.
Just because you see your login name, does not mean the launcher is signed into the same Microsoft account you used at the time of migration. Minecraft licenses are associated with the Microsoft account used to purchase or migrate the game, so you can have multiple accounts signed into the launcher at the same time, but if it signed into the wrong account then it will ask you to purchase the game.
To resolve this issue, ensure you are using the official Minecraft Launcher from Minecraft.net, and ensure you are signed into the identical Microsoft account that owns the license. Also check you are launching the right edition - Java and Bedrock editions are stand-alone editions, each needing their own licenses. If the "buy again" message persists, try uninstalling the launcher, restarting your computer, and installing a fresh Launcher. This often removes any cached log-in information.
If that does not work, Mojang Support can help confirm your license is associated with whichever account. You just need to use the email used during the migration or proof of purchase.
Best regards,