I agree with the 75k change you want to do. See the bullentin below - depending on your driving and other criteria the plugs may not make 100k. Add to this that the ignition has to work harder on worn plugs. I'd replace them with a new set (and wires too) and be good for a long while.
Service Information 2004 Pontiac Grand Prix | Grand Prix (VIN W) Service Manual | Document ID: 1809214
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#PIP3069C: Information - Iridium and Platinum Tip Spark Plug Maintenance - keywords 30K major minor preventative removal remove replacement tune up - (Apr 4, 2006)
Subject: Information - Iridium and Platinum Tip Spark Plug Maintenance
Models: All GM Passenger Cars and Trucks
with Iridium or Platinum Tipped Spark Plugs
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The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.
Condition/Concern:
The information below was originally sent to all GM Dealers on October 14, 1999 as a DCS message. This information applies to all vehicles equipped with Iridium or Platinum Tipped Spark Plugs.
Recommendation/Instructions:
Iridium and Platinum tipped spark plugs are designed to operate under normal vehicle operating conditions for up to 100,000 miles (160,000 kilometers) without periodic maintenance. As stated in the owner's manual, maintenance needs may vary because of all the different ways people use their vehicles. As a result, more frequent inspections and replacements may be needed if the vehicle is operated in extreme conditions.
When no engine performance concerns are present, Iridium and Platinum tipped spark plugs should not be removed for periodic inspection and cleaning of threads, doing so may compromise the spark plug's ability to withstand their corrosive environment. The threaded area, although not sealed, serves as a protective environment against most harmful elements. Removing and cleaning spark plugs will introduce metallic debris and brush scrapings into the thread area which may further the corrosion process.
Chromate coated spark plugs should not be wire brushed or handled in any way once they are put in service. Chromium topcoats form a protective oxide on spark plugs that is not effective if scratched. Both coated and uncoated spark plugs will have the best chance of surviving a corrosive environment if they are left in position. Attempts to maintain spark plugs by removing them and cleaning the threads can actually create the corrosive condition that the procedure was intended to prevent.
Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.
GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See your GM dealer for information on whether your vehicle may benefit from the information.
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