Don't confuse Paul's warranty service on a headset with Ron's upgrade from one piece of equipment to another.
In the case of warranty service, you can mark the product for export for repair, and keep all documentation of shipping it outside the country. When it comes back, you will have to pay any import duties/taxes to pick the item up, but you can then submit a claim for the amount and get it returned, based on your proof of previous export. The principle is that this item has already had taxes paid on import to Canada, so they are not payable again... It's just that the post office doesn't have the authority to review your proof of export and not charge you. You have to pay it and then claim it back.
In the case of an upgrade, even when sending an old item back to get a new, better item, well, that's taxable. If you're lucky, the invoice will only reflect the upgrade cost, so you'll only pay taxes on that amount and not on the value of the entire item. But you are bringing a new item into Canada, that wasn't here before... Ergo taxes apply.