My answer would actually be different for your two examples. That is, given no other indication in the score, when executing a time signature change from 3/4 to 6/8, I would keep the 8th note constant, whereas when changing from 4/4 to 12/8, I would keep the beat constant, making the 4/4 quarter equivalent to the 12/8 dotted quarter. But, if the 4/4 - 12/8 change was happening in a flurry of other changes involving 2/4, 5/8, 7/8, etc., it is far more likely that the 8th note is meant to stay constant.