Food

So I want to switch my dry food from the vet's Purina DM to something more diabetic friendly. I am either going to go with the Young Again Zero or the Dr Elisey's Clean Protien chicken flavor. He only gets a very little, anyways, but I would still like to switch. He mainly eats wet with added water.

How should I do this and work it in with his BG levels? How do I jnkn if it's good or bad? Do I just test an hour after food and monitor it for 3 days?