The lender still underwrites the new property and confirms you qualify for any additional amount you need to borrow under CMHC's stress tested standard, so porting is not automatic even when your existing mortgage allows it in principle, and a rate hold typically runs 90 to 120 days.
If the new home costs more than your old one, the extra amount is typically added as a blended rate combining your ported rate with a current rate on the additional funds, so the saving is partial rather than a full carry-through.
Confirm porting terms with your specific lender before you list, since timing between your sale and your purchase can affect whether the port goes through cleanly. Get your hand-reviewed valuation from Joel Dyck.