Switching milk!

So after a failed breastfeeding journey; I’ve now had to accept that my 3rd baby will be formula fed and have a little bit of expressed milk I have frozen each day…. So that being said; we have decided to switch from the Aptamil ready made milk to the Aldi powder formula. She has had 3 feeds so far and seems to be ok. But is it too soon to assume all is well? When people have switched milk how soon / how long has it been for there to be a reaction if any for their little ones? Any info / examples would be much appreciated x