I'm due somewhere at the end of August / start of September (waiting for a dating scan), and will have a 4 year old starting school full time as well as a 3 year old starting half days.
I'm not too worried about my 4 year old going for full days. I think she'll throw some wobbles about wanting to stay home and spend time with her new sibling (as will my then 7 year old, no-one wants to go back after the summer holidays!), but that's to be expected. I also think she'll be exhausted when she does come home, and will probably be a hand full of trouble until bed time ... but we can deal with that! I don't think everything will be too much for her, but she's been going for half days for a year now (18 months by the time she does full days). She'll know most of the children in her class already, she'll have met the teachers before, it's the same building etc. Kids bounce and take on new things easily though =)
As for my 2, then 3 year old ... I'm not sure. She seems so very young still at the moment and I just can not imagine her starting half days yet even without the introduction of her no longer being the youngest. I'm actually toying with the idea of maybe delaying her starting nursery and keeping her home with me a while longer.