dear son is around 6mths old, slightly younger than your baby.
i just asked pediatrician the same qns 2 days ago when dear son was having injections.
he said anything from 6mths onwards can start changing.
But he keep remind me NOT to mix the FM. cannot mix 2 diffe stage FM into the same bottle. he said can just change straight away but to play safe also can change slowly.
lets say your baby is having 4 milk feeds per day, start with 1 meal stage 2 + 3 meal stage 1 milk per day for around 3-4 days then slowly increase to 2 meal stage 2 + 2 meals stage 1 and so on....
dear son also dont seems to like solid food esp porridge... sigh~