I second that. Do a custom install from the original cdrom and install fx only. Be warned though, you may overwrite more recent fx with the older fx, so be sure to reinstall the newer versions after to make sure you have the most recent version.
I've installed older versions on top of newer versions of Sonar and encountered some conflicts if done in the wrong order.
cakewalk themselves always suggest installing each version in order to avoid these problems.
Anyone remember the classic reverbs and surround sound mixing of previous producer versions? I used to love those plugins.