I've been asking this question for years. Why do the rendered preview files disappear from sequences in the introduction of new versions of Premiere Pro? I need to spend hours deleting the gigabytes of preview files and rendering all files on dozens of sequences. There must be some way to reattach the preview files?
Sorry! This sounds like a bug you can file on User Voice, and a great question, but as to the reason, I am not sure. Re-linking preview files has always been rather dicey and I've talked to devs about it before (yes, it happens to me even in between new versions). I have heard it may be related to the preview file format. Which one do you use?
As an editor, though, I can advise to avoid updating any projects that are currently in production - especially if the new features might impact my workflow or if my project is particularly complicated, like a doc or feature. I don't like it, but I do it.
If I remember right, that's why Walter Murch was still working on Cold Mountain in FCP 1 or 2 by the time FCP 4 was out. Safeguarding that project file!
Typically, I wait until the new versions have been out awhile before beginning any new projects, preferring others to wring out the issues. Hope we can get your bug more attention over on User Voice. Premiere Pro User Voice