I switched since my baby is 9-M old, I switched because #2 contains more iron and calcium which according to some articles that I read is more critical to baby's growth as that stage.
Infant formula -- up to 9 months, but I started changing
when my sons were about 10 months, it is more safe,
actually, you can consider to change from 9 to 24 months
(this is what they said so)