I assume you mean IS 4.2 SP3, not 4.3. We had to do this in an upgrade scenario when the lower level environments were higher than production.
Regardless, it is possible to move rules between different versions. To do this you need to manually edit the XML in the Rules export ZIP file to match the target version. Then import as normal. I assume this is limited to functionality of the same version (eg IS_UNIQUE only exists in 4.2) and backward compatibility would only allow you to migrate so far. Anyway, if you must do it then it does work.
Or you can manually recreate rules.
regards
Adrian