You can choose to switch to lifetime billing at any time. Just visit your account page and select the lifetime billing option.
If you switch during your trial, you will be billed $699.99 when your trial ends.
If you switch during the first 30 days after a charge for a yearly subscription, you will only pay the difference, $580.11.
At any other time, you will pay the full $699.99 and no full or partial refunds will be granted for prior payments.