You can NOT buy Limesurvey. You can download it for free and install it on your own server or you can have a subscription with Limesurvey Pro. I will assume you are referring to the later. I am not sure if they can downgrade you, but you will have to ask them directly via the "Support" link in the top right navigation on this website.
I answer at the LimeSurvey forum in my spare time, I'm not a LimeSurvey GmbH employee.
No support via private message.
