Regular milking would nullify the magic as it would let the milk be exposed to air long enough for the effects to be negated.  If Raricow’s milk was extracted in a vacuum, like from it being gathered from a milking machine, then the magic would remain and those who drank it could experience changes.

Because there is only a little magic in the milk, it would take a lot for it to have a permanent effect on someone who drinks it, and if someone didn’t drink enough for it to be permanent, then they would go back to the way they were before, though they might be taller, or heavier in one manner or another than before.