It's a very good idea to replace all four rather than just two - especially one on each axle.
But - you should replace the dampers as well, especially if you are increasing the springing rate, otherwise the car will be under-damped and might have some peculiar characteristics.
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2011 XC90 D5 Executive
2003 C70 T5 GT
2012 Ford Ranger XL SC
1977 Triumph Spitfire 1500
1976 Massey Ferguson 135
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