Welcome to the community!
It's possible to migrate from a post-paid to a pre-paid account. As far as I am aware, the migration can only be done by contacting customer service to have them switch your plan types (see here). In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above. If your account hasn't already been changed, you might need to re-contact customer service.
You should note there would be a migration fee required which would be applied to your next post-paid bill.
Are you currently financing or subsidising a device? If you are, you should also note that you would need to pay the balance owing on the device when changing plan types.
Have you had a chance to familiarise yourself with Fido's pre-paid services? You should note that Fido no longer has a traditional pay-per-use (ie By the Minute) plan available anymore. You would need to select a monthly pre-paid plan (see here). You should also note that the top-up denominations do expire depending on their value (see here and here).
Hope this helps 😀
Good morning @Manisha5 , welcome to the community. When you called in to Fido to change your plan from postpaid to prepaid, I don't think that it would show up immediately. Were you told when to expect the changes? We're a community forum consisting of fellow fido users and don't have the capability to access accounts or make changes. You will have to contact Fido using any one of the methods mentioned Here and ask for clarification of the process. Hope you get the information you're looking for