If you have a seperate hatcher to move the eggs into 3 days before they are due to hatch then you'll be fine as you can leave the later ones in the inci - that's what I do and it works well as I can put eggs in as and when in the main inci and then move them into the hatcher as long as I leave a 3 day window for each batch to hatch so I can clean everything out! But otherwise as Chuck said you have to increase the humidity when the others start to hatch so could damage the others which are due later or could even kill them by drowning them in shell. Not to mention all the mess and gook the hatching chicks will make all over the other eggs which in turn could go nasty and smelly and kill them off by infection. If you've just got the one incubator then I wouldn't put the new ones in, get them when the others have hatched and you've sterilised the inci.