You can wait until 100K, though if you choose to change them now that would not be a problem. Due to the manufactors design unless this engine has/is being performance modified I would not deviate from the plugs that GM had put in at the factory. Though other plugs may give better performance at high engine speed they do not work well at low or idle speeds of the engine. You will notice some roughness while the engine is at idle and slight hesitation when applying the throttle. With the proper plugs you will get a smooth running engine at idle and proper performance under all loads and not spend a fortune on the premium plugs.
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