Can someone please remind me of what tag information other than settings I could/will lose (assuming I've already updated tags from library) so I can weigh the consequences or search the forums for ways to preserve that information? For instance, will my custom tags be lost? What about tags on video files for formats that don't support embedding? I think the xml files probably catch everything, but I've never checked thoroughly. I suppose all the RSS feeds will need to be re-imported, but that's not huge.
The Tree tags panel is useful to see which fields are actually written as tags into a file:
Tree > Tag Dump > Vorbis Comment Block is where MC writes most tags. However this view is only available for one file at a time. You may want to compare the Vorbis Comment Block to the tag panel list "
Fields With Values".
"Manage Library Fields" has a pull down menu with lots of useful views, including "Show only user fields" to get a list of the Custom Fields you have defined. Compare the list to the contents of the Vorbis Comment Block.
If some tags are missing from files, try checking "[ ] Save in file tags (when possible)" in the "Manage Library Fields" window, and then select all albums and
Tools > Library Tools > Update Tags (from library).
To examine fields across your whole library at once, use SmartLists. For example, you can search for files in which particular fields are empty.
You can also use the
Tree > Tools > Customize ... menu (small icon in upper left corner of the Tree) to create a condensed list of fields which you want to examine.
Note that if you defined tags in third party apps, such as Mp3tag, but have not defined them as fields in MC, they may appear in the Tree's Vorbis Comment Block, but will be otherwise invisible to MC.
Of course you should back up your old library. With
File > Library > Restore library... you can look at your old custom field definitions and settings if necessary, as well as review the old file data. You can also restore just settings (but settings alone does
not include Custom Fields, according to the wiki
https://wiki.jriver.com/index.php/Library_Backup).