On the back of the case, you will see a number of 'slot covers' or 'backplane covers'. Most MicroATX cases have 4 covers (some cheat and only have 3), and a true ATX box has 7, or occasionally 8 or 6, available slots on the back.
So long as you have the right port-cluster cover for your MicroATX MB, it will fit a full ATX case. You just move the standoffs in the case bottom around to fit. The three bottom-most slot covers are usable only to mount things like the second RS232 port, the joyport if it isn't on the port cluster, a USB or Firewire breakout module, or a sound card additional-port module, none of which actually plug into a PCI or AGP slot.
