I echo what Dyana said. The change to lower carb food will cause lowered BG so gradual idea would be a good idea, along with a reduction in insulin. If you were able to talk to the vet about dosage today, what did he think?
Purina DM, canned at least, is not terrible carbs-wise. The issues with it are the cost (lots higher than Friskees, Fancy Feast, etc), and the ingredients aren't as good or any better than most brands you can get off the shelf at the grocery store. The canned is heavy on liver and by-products, and many cats might eat if for a few days and then refuse to eat it anymore. It only comes in one flavor, and most kitties seem to want some variety. The dry, though, is
not good, just because it's dry food. I wouldn't even keep it around for "hypo" treatment. Good news is that you can bring it back to the vet. Tell him your kitty won't eat it, and they should refund your money. A better food to have around the house, in your "hypo kit", is some gravy-style Friskees or Fancy Feast canned food. Most times, people can just give a spoonful or two of the gravy to bring up the BG numbers quickly.
Dry food isn't good for any cat. Somehow, the people at Purina have convinced vets around the world that their "prescription diet" food is good stuff, because most vets send new diabetic cats home with either that, or with Hill's Science Diet (which is even worse food for diabetics). I and a lot of other people here rely on Dr. Lisa's information on Feline Nutrition heavily. Here's a link to her site:
http://www.catinfo.org/?link=felinediabetes. In a perfect world, it would be a required bookmark for every vet clinic!
Carl