I went ahead and replaced them on our 2014 Grand Touring AWD after almost 5 years and 60k miles. I know the manual says 75k miles but I figure it was time given the high compression ratio SkyActiv engines and the lots of stop and go seen by this car (my spouse's hour long city commute). I got the Mazda OEM plugs (PE5R-18-110) despite their high cost, given the experience of others on this forum. Very straightforward job: remove the cover, remove the bolts holding the ignition coils, unplug connector for each coil, and remove and replace plugs. After seeing them I'm glad I replaced them.