A retrofit conversion kit for converting a generally moisture intolerant door operating apparatus from overhead use to underground use and for operating a swinging door while protecting the door apparatus against moisture and dust encroachment within its underground location. The kit includes an enclosure to be mounted in a sub-floor stratum beneath a swinging door, with all of the door operating and controlling apparatus, whether electronic or mechanical, being self-contained within the enclosure. A seal arrangement provides the contamination encroachment protection while permitting access to the self-contained operating apparatus within the enclosure while the door remains in place.