Actually 1 year old babies can take stage 3 milks already:) if baby drank similac well, you might want to continue that. The official recommendation is that 1 year old children can simply drink cow’s milk already (provided there’s a well balanced diet) but there are many parents who do continue with formula
Yes, I changed when baby was around 1y3m.
If diary intolerance, try goat milk
Aptamil is ok for my baby