Welcome to the community!
Fido does not allow switching numbers from one line to the other as the number is tied to the plan.
If you just want the other number and not the plan and you don't want to keep your current number you can port the new number out from Fido then port it back to your line. Once you port the number out it will automatically cancel the line.
If you want to transfer the entire line from one account to another that you can do its called a transfer of responsibility. The person giving up the line will have to contact customer service and let then know and provide all the necessary details then you will contact customer service to finish the transfer. Fido also charges a $35 fee to do this, see here.