In general, "indoor" formulas are much like "lite" formulas... the biggest difference is that these often have more carbohydrates (usually in the form of grains) and more fiber. The way I see it, its actually just a gimmick, and an indoor formula is not actually better for an indoor cat. In fact, the higher amount of fillers and carbs are not a good part of their diet.
It is perfectly fine to switch to a different brand, but the transition should be made gradually, especially the first time. If you regularly feed a variety of brands, then switching between foods is easier. Switching between canned foods is easier as well.