There are some who argue that annual coolant changes on late model vehicles with bi-metal engines (aluminum heads/iron blocks) and/or aluminum radiators is a good idea.
It does not really make much difference how often the coolant is changed as long as it is changed before losing its corrosion resistance. Antifreeze is made of ethylene glycol (which never wears out) and various additives (which do wear out).
Some additives provide "reserve alkalinity" to neutralize internal corrosion before it can start. As long as the coolant is changed before its reserve alkalinity is depleted, can be expensive internal corrosion in the radiator, heater core and engine.
How can you tell when it is time to change the coolant? The only way to know if the coolant still has adequate corrosion protection is to test it. By dipping a test strip in the coolant and noting its color change, you can determine coolant condition and whether or not it is time to replace it.
When coolant is changed, the system should be reverse flushed rather than simply drained. This helps dislodge and remove accumulated debris and debris in the system. It also removes old coolant that would otherwise remain in the engine block.
Use of a cooling system cleaner is not necessary unless the system has been badly neglected and is full of lime deposits.
The cooling system should be refilled with a 50/50 mixture of ethylene glycol antifreeze and clean water. This provides freezing protection down to -34 degrees F and boil-over protection to 265 degrees F.
When coolant is changed, inspect belts and hoses. Make a visual inspection for leaks. Pressure test radiator and cap. Check operation of heater and defroster.
The thermostat does not need changing unless it has been causing trouble or the engine has severely overheated. If a thermostat is replaced, it should have the same temperature rating as the original. This is extremely important on late model vehicles with computerized engine controls. Fuel, ignition and emission functions are all affected by coolant temperature.