If the vehicle starts and operates without issues, I would be inclined to refrain from making any changes. If a plug becomes broken, it may be extracted by drilling, but it may not be worth the inconvenience if the engine is functioning well.
Two out of the six plugs in my OM642 engine have failed, although this has had no significant impact. Due to a malfunctioning relay, none of the plugs were operational, resulting in a delayed cold start. I used the vehicle in that manner for about a year without experiencing any kind of misfire. In addition to occasional plug failures and relay issues, I have also had a failure of the 225A 'mega fuse' in the glowplug circuit.