It took me about 20 seconds per side to replace the 535i style (chrome surround w/chrome faced slats) on my "Black Sapphire Metallic" vehicle with the OEM M5 style (chrome surround w/gloss black faced slats). Although I liked the original grills, I found the M5 style more understated in appearance with my dark vehicle color.
Since my vehicle has a full frontal clear bra on all painted surfaces, I was't hesitant to proceed by inserting my fingers in & behind the slats of each grill side applying steady, controlled, thumb pressure on the covered painted surfaces surrounding the grills, starting at the top & working my way around releasing a few clips at a time, thereby removing the grills. Replacement grills just slid into & engaged quite easily. Thank you DIY'ers from this & other forums who preceded me and gave me the courage to attempt this repacement method.
See happy face & one of my thumbs, upturned, below:
