>so maybe each time I go in the hive, replace a couple of frames with new frames with 4.9 mm foundation?
No unless they are very strong and those frames are empty. I'd just move the larger ones to the outside and, if they are really strong, feed the 4.9mm into the middle of the brood nest where the bees are more prone to build smaller cells.
>Will they generally draw it out properly, or is it still kindof hit or miss?
It depends on many factors from the bees, to the spacing of the comb (closer to 1 1/4" spacing leads to smaller comb), the time of the year (they seem to draw smaller in the spring), to the location of the comb (smaller in the brood nest and larger in the supers).
>i wish I had heard of this before I hived em
It would have saved a lot of steps.