From what I'm reading, your previous phone had both Fido and Koodo SIMs? I'm confused.
If you already have a new phone and just want to swap your hardware to use with your Koodo plan, than take out your Koodo SIM and put it in the new phone. If it doesn't fit (too big or too small), go to a kiosk that deals with Koodo or even order the proper sized SIM online. They're $10 + tax.
Now if you're talking about upgrading your phone, then the rep at the store will give you the appropriate sized SIM and activate it for the new device (if needed).