We changed from A+1 to A+2 when my baby was about 9 months old. My baby is OK when we changed from A+1 to A+2
When he was about 15 months, we want to change from A+2 to A+3 but my baby refused to drink it as A+3 is quite sweet and with strong Vanilla taste. When I called 美贊臣, 姑娘 also suggest the same method as mixing A+2 and A+3. We did that and it's OK. We later just changed to A+3.
Actually one doctor said that all formula milk should be similar. In western countries, there is no A+1,A+2,A+3 and A+4. It doesn't matter even if you change it or not but just that the infant formula is more expensive.
The other doctor said that A+2 can be fed until 3 years old. If my baby refused to change to A+3, it's still OK to continue with A+2. So, I guess it doesn't matter for you to switch to A+2 at the moment. Just up to you.