Import the pdf into CorelDraw. If it loks the same as it does when you open it in acrobat, then it's just a matter of printing separations.
If it doesn't then you'll have to use Indesign to separate it.
Are you sending seps to a printer? They should be able to separate it for you from PDF. Are you ripping it to your own imagesetter? Then print seps from Corel. Do you need to send separated files to a printer? Print to PRN from Corel.
There are a dozen variables we'd need to know to really help. What are your requirements in the end? Files? Negs?
I'm not sure I fully understand the assertions made in that post, but for certain, changing the Document Color Mode in Illustrator doesn't change the colors of anything that's already on the artboard....
About the worst thing you can do with Illustrator is use the Color Mode to change stuff after starting your design.
ALWAYS select your document color mode before you begin using Illustrator. Changing...