Recently I needed my entire library to change the tag UNSYNCEDLYRICS to just LYRICS.
so I selected all files, clicked Extended Tags, clicked on UNSYNCEDLYRICS, renamed it, clicked OK.
I thought the program understands my intention, did all files and said SUCCESS.
unfortunately I should have checked what MP3Tag really did before closing it, because all my lyrics tags have vanished. None of my stuff has lyrics now and days of work is gone.
my bad for trusting MP3Tag blindly, but this should definietly be looked at.
The procedure works for a single file or all files that have the same data in the original field as the value from the original field is taken and also displayed.
When you set a new name in the field selector, then the already displayed data is used to fill the new field.
If you select files with differing contents then there is no data to be displayed that could be used literally but the varying field contents is displayed as <keep>.
This is also represented in the extended tags dialogue where it says now <blank> for the original field and <keep> for the new field.
If you now close the extended tags dialogue, then the original field is deleted as there cannot be empty fields and the new one is created with no contents as it has no data to <keep> and therefore deleted straightaways as there cannot be empty fields.
If you want to transfer data from one field to another then basically, there are 2 ways:
Try Convert>Tag-Tag for TARGETFIELD
Format string: %sourcefield%
and then delete SOURCEFIELD.
or
Try and action of the type "Guess value"
Source format: %sourcefield%==
Target pattern: %targetfield%==%sourcefield%
This assignes the data from the sourcefield to the targetfield and removes the sourcefield in a single step.