When you purchase a license, you'll get a new license key. Then, just update it under Settings > System > General > License Key. This is the same regardless of who you purchase your license through. If the license has already expired, when you try to login it will prompt you to enter a new key.
Licenses are locked to the hostname, IP and install directory. So, if you install at billing.domain.com, it will be locked to that location. However, you can re-issue your license anytime if you decide to move it.