I agree. Our first went to morning preschool, the second to afternoon. Each were 4x/week. And I much preferred the afternoon! He still napped, but he just moved his nap until after school and would doze for an hour or so. We loved having the mornings to laze about and play together!
Have you noticed cliques at this age elsewhere? My kids had new children in their classes every year, as well as new teachers, and they just made new friends with whomever they met. They "kept" their friends from previous years and might see them outside of school, but kids at this age are great at making new friends where ever they go! So, if you do decide to switch, just let the teachers know your concerns and they can make sure he is introduced to each child and finds some little buddies right away.
Good luck!