Welcome to the community!
The answer to your question depends on the type of plan you have. If your contract is nearing its end (or has ended), it's likely you have a subsidised plan. You should note that Fido has recently switched from their old subsidy model to a financed Payment Program. With the subsidised plans, the cost of devices was incorporated into the cost of plans (ie Small, Medium, Large, etc). With the new model, the cost of devices is separate from the cost of plans.
If your plan is a subsidised plan, your monthly billing will not automatically be reduced once you have paid the balance of the phone. It is not possible to simply remove the subsidy from older plans so you would need to switch to a new plan. However, you should note that the details of the new plans may not be the same as your current plan. If you did not switch plans, you will not automatically get a new cellphone. In order to finance another device, you would also need to switch to a new plan as the old subsidised plans are not compatible with the new financed model.
Hope this helps 😀