Originally Posted by whoover

Just don't lose the old key. If you didn't understand my earlier question, note that if the keyless entry and start doesn't work (perhaps because of a dead fob battery), there's a failsafe mechanism. The old way is to pop out the start button and insert the rectangular part of the key into the dash. It works without a key battery because there's an infrared-based key validation that uses only the car's power. The new way is to place the dead key on an NFC pad in the console. Near-field communication works by using inductive power from the pad, and doesn't need any key power. (This is how NFC point of sale credit card taps work.)

So the new key won't work with the old dash, which means any keyless start problem will leave you stranded unless you have an old key that will mate with the car's dash.

Originally Posted by trustz

I did a conversion on my 2014 W222 to 2018 looks inside and out.
Now I am going to have my old key upgrade to a 2018+ key AMG style: