I make water kefir but would like to try milk kefir, can I convert my water granules to milk kefir granules?
It might be technically possible (with a lot of work) to get water-kefir "grains" (or, more correctly, SCOBY, a Symbiotic Culture Of Bacteria and Yeast) to survive on milk, although I'm not sure what kind of drink that would produce.
But if your question is, as I suspect, whether you can turn the water-kefir SCOBY into what people commonly refer to as "milk kefir", then the answer is an absolute no because they are completely different cultures.
This is a common point of confusion, and it arises from the fact that "water kefir" is a misnomer. The word kefir properly refers to the culture and related drink based on milk, and it probably originates from the Caucasus area.
On the other hand, what we informally call water kefir is actually a completely different culture and drink, more correctly called tibicos, and it probably originates from the Americas.
It probably won't work well, since you'll be missing the microorganisms that feed off milk sugars and what not. See this link for more details.
Just buy some milk kefir grains rather than wasting milk.
I'd guess that if you're starting with Lactaid, then water kefir grains may work better than they would in regular milk.