Sorry if this is a late reply, but here goes;
As was said earlier, the time needed for plants to produce enough carbohydrates for an animal to live a short time on makes it more or less impossible to have any form of autotrophic animal.
I won't even get into the area needed for photosynthesis for producing any significant amount of energy compared to the animals weight.
I'm also going to go out on a limb and say that the metazoan lineage has never possessed any genes for photosynthesis. Litterature on the evolution of metazoan and plant cells mostly agree that what we know as plant cells formed when eukaryotic cells for some reason started an endosymbiotic relationship with photosynthesizing prokaryotes (now known as chloroplasts) AND heterotrophic prokaryotes (mitochondria). Animal cells on the other hand only had, and still have, an endosymbiosis with heterotrophic prokaryotes. To this day, the photosynthesis of plants is strictly bound to the chloroplasts.
So I'm afraid the answer to your question about de-evolution is that it won't get us something that we never had.
Edit: Bah, had so many interruptions when writing I sort of forgot a bit of the post that I was replying to
. Anyway, trees could probably evolve towards simpler forms provided that there are some free ecological niches available. No doubt some other, already simpler autotrophic organism would get there first, but it is concievable.