Cars with high mileage must have their transmission oil changed on a routine basis per the manufacturer's specifications. This will prevent burnt sludge from collecting inside the transmission, causing the internal components to wear faster. Routine transmission fluid changes also ensure that sludge, grit and microscopic metal particles that develop over time are safely washed away before causing any damage.
If your car has high mileage and the transmission has not been well-maintained, you should not change the transmission oil to suddenly adhere to the suggested routine maintenance schedule. The dirt, sludge and grime that has built up on the transmission plates over time is actually assisting the clutch plates by adding additional friction to the components. If you don't have problems, don't change the transmission oil.
Suggested routine maintenance on your car's transmission is the wisest course of action to consistently follow. However, if you own a high-mileage vehicle that has not been well-maintained and you decide to change the transmission fluid, be aware that the fresh fluid viscosity will greatly magnify any transmission problems that may exist. This could lead to transmission failure in the near future.
