Let's have a look at the Danger zone that you'll find at the bottom of the project description in your dashboard if you are the project owner:
Transferring a project is useful when you want to start a new billing cycle under a different account. In this case you can go ahead and click "Transfer".
This will let you input the email address of the destination account.
Conversely, the receiving account will receive an email and find a popup at the top of their dashboard:
On accepting the project you'll be charged for the extra project if you are exceeding the limits of the free "Developer" plan.
If the former owner is left with any unused credit it will show in their billing profile and those will be used automatically on any new invoice.
Credit cannot be transferred from one billing profile to another.
When duplicating a project you'll be asked if you want to duplicate the schema only or also the data by using this toggle: