How do I switch from my monthly plan to prepaid koodo plan , also I would like to keep the same phone number
You'd have to contact prepaid customer service. Any tab balance that is left will be billed on your final invoice plus any overage if any. Your post paid account will be cancelled right away and be effective on the prepaid side. Make sure you have your $20 prepaid sim card handy so you can activate it on the main Koodo page. Click activate prepaid phone on the bottom.
Since you want to keep your phone number, tell the post paid department that - they will ask you several questions about your monthly account (like the pin and the account number). This will allow them to move your number (also known as porting) from one provider (Koodo postpaid) to another (Koodo prepaid).