You need a slim socket to change plugs on the 535. I know Assenmacher and Burger Motorsports have them, maybe others.
Cylinder 6 is tricky as it is below the upper strut support bar. Requires a swiveling ratchet head and universal joint on the extention. Otherwise straight forward.
Reason to replace them is to avoid misfires. Ignition coils also have a tendency to get worn out at 75-100k miles.
Audio: Jehnert Flatline speakers, Mobridge M1000 DA2 prepro, Helix P-DSP, Zapco Z-150.6 amp, CIC iDrive upgrade
Performance: Cobb Pro Tune, AR Downpipes, ER FMIC/CP, DCI, RB turbos and inlets, KW V2 coilovers, Fuel-It Stage 2 pump w Ported fuel rail, Mfactory LSD