The issue isn't if they are modified or if the new version is written from scratch, the issue is what things that worked on the old version break on the new version.
If few things break (Windows 3.11 to Windows 95) you get nearly everyone to upgrade (unless the hardware can't run the new version at all, mearly running it poorly didn't stop a LOT of people from upgrading)
where it breaks things, including the user interface (Windows XP to Windows Vista for example) the uptake of the new version is much lower
the fact that KDE4 is a re-write vs KDE3 is a good thing, right up until you state that a feature is lost because the developers didn't care enough to make it part of the re-write, and that it's up to the users to fix this (if it was merely missed because nobody cared enough to notice during the re-write, then reports from users that they cared about the feature should get it on the list of things to fix, it may take a bit of time, but KDE 4.x has had time now)
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