If your license isn't valid per year, then yes, the serial number should be valid only for the version of the software purchased then. For newer versions of the software, it's possible that you will have to buy the product again.
On a side note, I have checked the official website, but the domain has expired making it impossible to buy or download the software. If you have the CD, then you can use it to re-install the software and use the serial number attached to register it.
