I've already flipped DD's. She wouldn't sleep in her crib and I thought, maybe, the mattress was too hard since she was used to our soft mattress so I flipped it to the softer side. That being said, it made no difference for her and she's back in our bed until we return from Missouri so I can sleep!
"To be able to practice five things everywhere under heaven constitutes perfect virtue...gravity, generosity of soul, sincerity, earnestness, and kindness."
I would think if you feel your child has pretty good head control you would be fine. I think it is really about knowing your kid and his abilities to feel safe doing that.
Re: flipping the mattress