Yes, certainly Dwight, just activate your prepaid account and port in your postpaid number :)
Your postpaid will be cancelled automatically, and all you still need to pay is the remaining bill & tab (if applicable) :)
Is your current number with Koodo Dwight? If it's a Koodo issued number, I see no reason why you could not port it out to prepaid (or anywhere else). You can always try it, you can do your prepaid activation all via Self-Serve. It's a 5-minutes deal and really easy!