Пишем в службу поддержки (Support) следующее письмо:

Hello. Please change my UserID from *название старого* to *название нового*. Thanks.
- Last 4 digits of the credit card number:
- Paypal transaction ID:
- Account:
- Secret question and answer:
- First and last name:
- Date of birth:
- Any product code attached to account:
- E-mail address registered to account:
- Payment Method:
- Last 4 digits of the credit card number:
- Paypal transaction ID:

Категорию запроса выбираем Account/Billing, I need help with something else.