That is exactly why - because you changed the motherboard.
I have a z270-a too. Din't bother to check the pcie order....Why would you need it? You can see and configure everything you need based on the dashboard information.
3x armor 570 hynix
1x armor 570 elpida
2x pulse 580 hynix
1x pulse 580 elpida
1x pulse 570 elpida
Damn, that is a complex rig I would suggest, maybe adjust oc settings slowly until you find the perfect setup.
One tip is: From what I saw, first 3 PCI-e are 16x ones, taken from the CPU down. Rest, I don't know.
Starting from CPU down I have: GPU3,GPU0,GPU5,GPU4,GPU,GPU1,GPU6,GPU2. GPU7 is in the m.2_1 port. Hope it helps