JUST CHANGED OUT THE STOCK IRIDIUM TIPPED PLUGS IN MY TOYOTA SIENNA MINIVAN AND WAS WONDERING IF I SHOULD EXPECT A DECLINE IN GAS MILEAGE/ PERFORMANCE? ALSO, WOULD I HURT THE ENGINE BY DOING SO?
THE PARTS STORE HAD A SPECIAL ON PLATINUMS THAT DAY AND COMPARED TO THE COST OF IRIDIUMS I FIGURED I SAVED ENOUGH FOR A WEEKEND BEER BINGE! 
Drink your beer in peace, don’t really think it will make a difference. Don’t drink and drive; Thanks
You’re supposed to check before buying. Here you luck out. Your new plugs will work just as good as the old.
The Iridium plugs have a longer service interval than the Platinum plugs. In the Toyota, it’s often 60K vs 90K.
Assuming you bought the properly spec’d plug for your car (heat range, etc), then the only difference you’ll see is with the service interval.
Did you buy Denso or NGK platinum? If so you should be absolutely fine. If you bought Champion, Bosch, or especially Autolite then you MAY see some roughness in idle and lower gas mileage. Then again, they may work just fine.
Platinum or iridium make NO DIFFERENCE in performance or gas mileage. They will just last longer. Platinum plugs can easily last to 60k+ miles while Iridium are suppose to last up to 100k miles. Personally I’d NEVER leave plugs in there that long…
Oh, and fix your capslock key.