Formula

I have a 9 week old he was on SMA Gold prem 2 from 3 weeks old his consultant has advised he doesn't need to be on it anymore and can switch to any formula I had already brought the aptamil he has been on it 6 days and he is constantly having really bad runny poos is this normal or should I change the milk again?. Worried what if i havent let him be on the milk for long enough as i read it can take up to two weeks for his belly to settle but then im worrired about his poor belly and if he is keeping anything down also if I change it now he will have to go through having a bad belly again. Any advice is very appreciated. Thank you in advance.