Hi @shoutforkrishna !
Through Fido, devices can only be reserved when they haven't yet been released.
If you purchase a new phone before the return for the previous one has been processed, you'll unfortunately be charged the full price.
That said, another possible option for you would be to get the new device straight from the manufacturer or a third party location (like Best Buy, for example) and get a reservation or a financing option from them.
You'd still be able to pair that phone with your Fido line!
Hope this helps.