lens hardware changes

I'm curious to know if Canon change the lens hardware to improve the optics, IS, etc over the life time of a lens. In other words is 70-200 IS 2.8 lens date code UR very different than the UV or UY? Does the used price would be affected too?

BTW, I'm not taking when changed from mark I to II, just the same version...

My experience is with semi-conductor production lines in Japan, and once they get a process dialed in and producing the expected yields it is not changed. I've read of minor differences in Canon lenses, but I don't recall them being major improvements until a new version is introduced, i.e. v1 to v2. Used prices based on date codes is meaningless IMHO, condition and use is much more important to me than a date code.