My point of changing would be as norm. with a regular seed......MH to get the growth before flowering and the HPS to get dense bud! I know some guys use HPS from seed to harvest,but i just thought someone may have expirimented with an auto strain. So i guess you use the HPS from seed too for your auto?what would your point be for changing i would hope not to elimanate stretch
i use flouresent tubes first two weeks then hps till finish for autos
for reg weed i sometimes use hps and mh rotating plants under both lights
or i sometimes use mh first two weeks ( for a big stretcher) then hps for good solid tight buds then switch back to mh ( for final bud groth stage)
for autos i pick a light go straight through with it
they say mh will give you fluffyer buds that appear bigger and hps will give you tight hard buds that are denser heavyer
if you have both lights try agrow with each see which you like best
good luck
1Luv