Totally! the visual aspect of the upgrades is super important, because it doesn't feel as good to watch numbers go up as to see an actual change in the units behaviors. I think that changing the guns the units handle could be a good idea, because i could change the gun sprite without animating or changing the units much, like the laser example. In the flying units case maybe i could give them a bigger/ more badass jetpack when they level up, making them faster and deadlier.
Don't mind the input at all, i could be talking about game design all day to be honest haha