Exhaust pipe replacement

If the existing vent is not the correct size or type then yes it will need to be replaced. If it is system 636, correct size and type then you can reuse it as long as the condition of it is acceptable.

System 636 exhaust pipe should be replaced when replacing the appliance. While some local regulations may allow existing, certified ULC S636 venting to remain under specific, inspected circumstances, it is highly recommended and often required to replace the entire venting system to ensure safety and compliance.

yes, with all newer water heaters you will require special venting like system 636.