I think you're safe to flip to the plusher toddler side any time after your LO is a year old. I didn't get around to it until she was 15 months or so, but it does seem like it would be more comfortable.
Not all mattresses have infant/toddler sides- ours came labeled with stickers, but there was a noticeable difference in plushness between the two sides.
I've flipped his mattress before but I never noticed it had 2 sides (toddler/infant). I'll check that today when I change his sheets. The kid is a peeing machine lately! Ugh!
Re: Mattress Flip
"I will show you the kind of big sister I will be..."
Not all mattresses have infant/toddler sides- ours came labeled with stickers, but there was a noticeable difference in plushness between the two sides.