kruz-
I agree with you on the continuous evolution of modifications and upgrades to favorite pieces. I don't know how many different dials, movements, crystals, bezels I've been through on my favorites, but I'm always looking for improvements where possible (though in many cases it's hard to beat genuine parts! )
tmark-
It's always difficult to say how long certain versions stay in production. A lot depends on how popular a specific model is. Seems that the mainstream models like the Subs, Seamasters, PAM's, etc are always cycling through evolution, and it's not uncommon to see a lot of overlap with watches of various points of origin and creation, while the more unique pieces like the IWC's, may not see any revisions or updates for a very long time.