I already have a Koodo SIM card but it's not prepaid. Can I use the same card or do I need to buy a separate one?

Is it a one time purchase for a SIM or do you need to purchase one for every 30 days cycle?

You don't need to buy a SIM every 30 days. 

Buy a prepaid SIM and activate it with your desired base plan. Then you just need to add funds to your account every 30dsys to renew.

If you want to avoid unnecessary contact, search for Koodo prepaid SIM on eBay.ca (not eBay.com).

I would like to do the exact same thing. I just received my prepaid SIM card ($10 on amazon.ca). It looks like I can sign up online for a prepaid plan but how do I make sure it transitions over from my existing Koodo monthly plan? I want to make sure I don’t end up with two plans under Koodo. I want to montlhy plan to end and the prepaid plan to start. Thanks.

If you want to keep your number:  Activate your prepaid SIM first with random number.  Then port your current number from postpaid to prepaid later.  Once porting is done, your postpaid account will be closed. You can access to your postpaid self account for 90day after the cancellation. So, you should check if anything left to pay or overpaid. 
 

If you want a new number :  Activate your prepaid SIM .  Contact Koodo to cancel your account. ( It doesn’t cancel automatically, so you need to talk to them).  You will receive final bill, but check your self serve is always good to make sure everything is done.

The porting plan Mayumi outlined is your best bet. You get to keep your number and you don’t have to fight customer scare to cancel your post-paid. Activate with a temporary number and once you are satisfied that things are stable, port the old number over. In the meantime, you can have the monthly phone with the old SIM installed forward your calls to the new number then go with the new SIM permanently. If you anticipate a large volume of calls, investigate the service charges for forwarding. If there is a monthly forwarding add-on for you post-paid plan, it’s worth doing, because pay-per-use gets expen$ive very quickly.

Best time to do the switch is a day or two before your monthly billing cycle date. You avoid wasting data, minutes etc that you have already paid for since the pro-rata charges will be almost nil on cancellation.