Anyone who replaces glow plugs one at a time has never owned a 603 or 606!
Getting to the stupid things is a job in itself on a 603, so all 6 should be done at the same time.
Now the 606 is a different animal, they are fairly simple to get at but have a habbit of breaking off. If I owned a 606 I would remove the plugs and ream the holes once a year. I would replace those every few years, or when they fail because they are somewhat simpler to get at.