I think energy would be the biggest thing to take into consideration. Adult foods have slightly more calories than senior formulas. Since he's not a very active dog, I'd probably go ahead with switching. I know a few people who fed their dog senior food until they lost the weight and moved back to adult though.

Kaedyn *could* be classified as a Senior (roughly 6 or 7 years old), I guess, but because of his energy levels, he's still on the adult food.