You can change the owner of amasty.com account by editing the email address on file. For that, log into the necessary account and proceed to Account information tab. There, click on Edit link:
Enter the email address of the new account owner and your current password, and then press Save button:
An email will be sent to the old account owner. In order to finish the process, the old owner needs to confirm email change:
After pressing the button in the message, you will be redirected to amasty.com website. Email address change is considered complete once you've seen the following success message:
Now the new account owner can log into it using their email address. They may enter the same old password from the previous owner, or reset the password via a
dedicated form.

In case full ownership change isn't required, but you want a certain email address to receive invoice information from the account, you can add 1 additional invoice email at
Account information tab. This email will receive copies of generated invoices:

Please note that the invoice email
can't be used to log into the designated amasty.com account. Customer accounts are identified by primary email addresses only.