You occasionally get situations when the animators switch the art style every now and then, but the characters still exist in the same universe, so I wonder if the characters wonder why everything looks slightly different.
Fair, there are several examples of characters acknowledging the shift in very specific circumstances. Often said circumstances are plot related one way or another.
But usually that's not the case, and if you have a change of art because now it's a different studio producing the animation or they release a new season with a different art director and budget then no, it's implied that the aesthetic status quo has always been the same.