Long story short: A Premium account is a legacy account and the only way to change the username is to migrate the account.

I have tried to give you all a head start on how to change your usernames and what issues you may run into. Hope to see more of you one the server.

Minecraft account - paid (formerly known as premium) 

• You have purchased a copy of Minecraft from Minecraft.net.
• You use your Minecraft username to log into Minecraft services, and can access the full game.
• You can reset the password and migrate the account to a Mojang account by visiting account.mojang.com/migrate .

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If you have a old Minecraft Premium Account you can migrate it into a Mojang Account.
Just enter your Minecraft account information.

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Changing your email address (legacy Minecraft accounts)
In the event that you do not have access to your old email address, and you do not have the password for your Minecraft account, locate the transaction ID or gift code from when the account was purchased, and contact customer support. Please provide the transaction ID, the old email, your new email, and we'll change it for you.

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Account migration issues

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Minecraft Login Issues With Migrated Accounts
Here's what I did, and how I fixed it:
• I migrated my Minecraft account to a Mojang account by going to account.mojang.com
• Afterwards, I tried to log into Minecraft.net with my username, and it made me use my email
• I tried to log in with my email, but it said invalid username or password (I think I had selected two different passwords)
• I used the forgot password tool on Minecraft.net , and it said "User not found"
• I went back to the Mojang account site and used the forgot password tool there
• I got an email from Mojang with a link to click, which let me reset my Mojang password, which also reset the Minecraft.net password

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