I think you are still doing things quite hastily. Naturally, you should first test this kind of procedures with a couple of files and a small test library so that you can easily try several different ways to do things. You could probably have avoided the whole problem by running some tests first. Now you still continue doing big moves without knowing what exactly will happen.
Last time I screwed up it was because the Auto Import did not work as I though it would. That's why it messed things up so good, with alot of files.
Normally I would experiment on a small set of files, but that does not mather much now. I should have the backups I need.
If you had searched the forum you might have found my posts about how the mpl file must be imported twice if it adds unimported files to a library and the intention is to preserve the original Date Imported values. The first MPL import populates all field data and after that MC marks the newly imported file as actually imported. This changes the Date Imported value to the current. However, if you import the MPL file once again the latter does not happen (since the files are already imported) and the Date Imported field gets updated from the MPL file.
Also, if this is wasn't already clear, only the Import Playlist tool in the File menu can fully update existing library files. The other import methods are designed to be less powerful to avoid accidents.
EDIT
If the filenames in the MPL file match exactly the filenames you have in the library then it should update all field values from MPL on the first import. If the filenames don't match it will either import nothing (if there are no physical files to import) or it adds new files to the library (if the named files exist).
You're right. I'd probably find it if I had just looked. Sorry about that. Sometimes I simply forget to do it.
I'm using the Import Playlist tool and it works as you describe. Import twice and the Import Date from the playlist is also updated.
Just loaded the new library backup and imported the old MPL file over it, twice. I ended up with almost 95% double tracks in the library again.
Why are they added again when no tag except the import date it different? Any good reason for this?
Anyway. I was able to seperate the old tracks from the batch with new albums I added only minutes later. So I deleted the old tracks, and ended up with the exact number of tracks as I had in the new library earlier. Yes!! I'm saved.
Thanks alot for the help Alex. You're pretty patient