Welcome to the community!
Do you have a prepaid or postpaid account?
If you have a prepaid account, you should be aware that there's a charge for changing your phone number unless you're changing it because you're moving to a different region (outside your current local calling area). For more information, you can visit here.
Hope this helps
I just got an update on that and changing the phone number online is also free for prepaid customers. Fido.ca will be updated with the info.