I don't think you need 1A03 as a pre-requisite to 1AA3. 1A03 is not being offered next term anyway, so you would have to take 1AA3, and you probably can but you might want to double check that.
I am taking 1A03 right now, and from what I hear they are different. In 1A03, you study art 'thematically' so according to themes like Sculpture, Printmaking, Painting, the Creation of Art/the Destruction of Art, etc. In 1AA3, you study probably similar themes but within more of a historical context. So whereas with the first course you don't have to memorize much except for a lot of terms relating to the themes, in the second course you probably have to memorize a lot of dates and have a good idea of how the art relates to historical events/periods, important artists, a lot more.
Sorry I can't help you much because I haven't taken the second course yet, I will be taking it next semester. But I expect it will be a lot heavier, a lot more memorization. I am not sure of the breakdown, but it could be similar to the first course which was one major paper, midterm and final exam. If you're interested in art, I would recommend taking it, I am sure it's good.
And probably easier than Econ, haha.