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.
Help us to help you!
Provide your LS version and where it is installed (own server, uni/employer, SaaS hosting, etc.).
Always provide a LSS file (not LSQ or LSG).
Note: I answer at this forum in my spare time, I'm not a LimeSurvey GmbH employee.
