Whats the problem?
You don't need to replace all of them. They either work fine, or they don't. Don't bother with replacing another until you get a misfire. Once you do get a misfire, switch a couple COP's to see if it's actually the COP that is failing and then make your purchase. If it's not the COP, look at plugs and injectors.
I would change them all then you won't have to worry about a misfire again if you just replaced all your plugs. You can get all 8 on eBay and they work just as well as the $42 ones from auto zone, but put a new fuel filter and get some mass air flow cleaner and spray off the MAF.






