It is not just about the ability of the coolant to not freeze. Unless you have added water to the system to top it up the protection temperature should not change.

I am no expect but have been lead to believe that the additives that provide internal protection in the system as "used up(?)" in that two year service period. This protection may not have been as acute in the past with all cast iron material, but the addition of an aluminum head on a cast iron block brought a different level of chemistry into play. Having personally found electrolysis damage on two different heads on cars that have wandered in here I make it a habit to keep my coolant changed every two years.

Every other year I change my coolant and flush the brake systems on our cars; cheap insurance.

Randy