This question popped into my head for some time.
Sure, but it’s going to be adjusting many things–for example code can be optimized so that you can do more with the graphics or you can simply make nicer looking assets without changing the texture resolution or polygon count.
It really depends on what the performance currently is, and the state of the existing assets. (i.e. how good do the assets look at the moment?) All in all you won’t really know for sure until you test things out.
Good assets and art direction is 80% of what makes good graphics. Baking things down and making things less dynamic can also improve quality and performance at the same time.
It’s called optimization. You just optimize visual quality instead of performance.