How to convert from MailerLite Classic to MailerLite (new version)

Create a new version account:

• Click on the Mailerlite new version
• Click the New Account button
• Name your account 
• You'll be prompted to migrate, but you'll need the API key from your classic account.

Get the API key from your Classic account:

• Click your account name in the top right corner to open your account menu
• Got to integrations
• Click the use button beside Developer API
• Click the copy button to copy the API key

Go back to your new account and follow these steps:

• Add the API key and click on Migrate. This will migrate your account details.
• Click on the Migration tab to complete the rest of the migration.
• Proceed with each of the migration tasks.
• If you are on the free plan you'll be able to migrate up to 1000 subscribers.
• There may be some areas that will need some manual work to migrate properly and some things will not 

Mailerlite also has this useful video where they lay out all the steps