tunings and string gauge

I have a 1974 Homer Ledford dulcimer and a 2019 Heritage dulcimer.  The Heritage is in just intonation Dad; I'm not sure if the Ledford is equal or meantime intonation. String gauges for both are 22-12-12 now.  Do I need to change the gauges to tune down to CGc on these instruments?  Is the just tuning tied to the key of D? Also, does lowering the action change the intonation on these instruments?