The Fat
The first thing that is noticeable when comparing brownie mix to cake mix is the quantity of fat. Brownie mix calls for much more fat to be added to the mix, which is one reason brownies are more dense than cakes.
So to make a brownie mix lighter, we need to cut or replace some of that fat. One way to easily do this is to use unsweetened applesauce rather than oil or butter.
Applesauce will act in the same moisturizing way as oil but without the heaviness. So bye bye oil, hello applesauce.
The Eggs
Adding eggs are going to help with the texture of the mix as well. Adding one more egg to your brownie mix with instantly make it less chewy and more cakey when baked.
So get out those eggs and if your box of brownie mix says to add one egg, go ahead and add two. The texture is getting more cakey already!
If you prefer not to use eggs, you still have plenty of options!
The Leavener
There is probably already baking powder or baking soda (which aren’t the same thing) in the brownie mix but to make it more cake like, we are going to need to add more. A teaspoon of baking powder is all you will need to take the brownies up to cake height.
Adding baking powder will make the mix rise more as it bakes and make it much fluffier than any brownie.
Add the baking powder to the dry ingredients of the box mix and whisk it together before adding the wet ingredients. This will make sure you have no baking powder lumps in your batter.
Brownies also usually tell you to add water, I would use milk instead, with the extra egg, and baking powder, it will have a richer taste anyway.I forget which but the box should say whether to add 2 or 3 eggs according to whether you want the brownie fudge like or cake like. I think it's three for cake like but check to be sure.
I'd probably use a bundt pan because it'll be less batter than normal. I might add a tsp of baking powder too if it were me.