wondering if i should switch formula?

Baby has been on similac advanced since the day after he was born (he is 8 days old)but has recently been very fussy and seems uncomfortable after feeding. He will also cry like he's still hungry after finishing the bottle but if I offer more, he will drink for a minute and then cry. I'm wondering if I should switch to similac sensitive or if there may be another issue since he's been on regular similac for this long. How long does it take for them to have a reaction if they are going too?