Lets look at this here.
A V Power cost under 3 dollars a plug.
So 3 x 8 = 24.
So behind all logic, swap all the plugs at once.
Platinum and Copper have different properties.
Platinum plugs retain heat burning themselves clean, thus giving a greater chance for detonation as they are hot longer.
Copper dissipates heat quicker. They don't last as long due to this feature.
Do a simple search on copper vs platinum and you will find a lot of information on this subject.