I successfully upgraded from MJ8 to MC12. Then I tried to import my MJ8 library from MC12 by using File > Library > Restore Library and selecting my backed-up MJ8 library.
It prompted me to do the conversion, as I expected, and all 6146 files are there. The only problem, but a major one, is that many of my files have titles, artistes and filenames that contain accented characters since most of the song titles are in Spanish: á, é, í, ó, ú, ü, ñ. For some reason, MC12 encodes these in some non-standard way (or perhaps MJ8 did). At any rate,after the import, all these non-7-bit-ASCII characters show up as an "unprintable" square in the MC12 display, including in the filename filed.
Worse yet, it's not just the way it shows up but in the way it's actually encoded, since if I try to play one of these files, it thinks the file doesn't exist (files that happen to have no accented characters play fine). Going through thousands of files to edit each one manually is not an option. More disturbing is the thought that if I update my MP3 IDv1 tags from the MC12 library tags, I could corrupt those as well.
If there is another way to import the library that bypasses this bug, please let me know, otherwise I will have to revert to MJ8 until it is fixed in MC. Thanks.