You can change your subscription at the original sign-up page. Make sure you are logged into your Rentometer account and then go to the sign-up page. The link to the sign up page is here: https://www.rentometer.com/pro
Once on the sign-up page, just click "Sign-up" button under Pro Lite and input your credit card information. This will change your subscription from a Pro account to a Pro Lite account.
Also, since you are already a subscriber, it will only update your subscription info and it will not create a second account.