It is currently only possible in JQL to compare version fields to a specific version, or to the functions releasedVersions() and unreleasedVersions(). It would be helpful to be able to compare one version field to another as in:

fixVersion != plannedFixVersion

In that example, a single version custom field (plannedFixVersion) is being compared to a built-in version field. This could be used to chart issues that were completed in a later version than originally planned (for example).