I would like to copy the BPM field to the fBPM field in my Mp3s via batch, As the fBPM is a custom field provided by Traktor DJ studio and the entry is wrong..
Is there a way to copy the BPM field overtop if this fBPM field?
Thank you! I've been looking for something that does this for ages!
You can create a new action.
Action type: Format value
Field: fBPM
Formatstring: %BPM%
This didn't work, as the fBPM field is case sensitive and mp3tag only creates uppercase 
You can open the *.mta file with a text editor and change the spelling.
Or here is the action that can add the desired spelling:
Action type: Guess values
Source format: %bpm%
Guessing pattern: %fBPM%
If there is already a fBPM field in upper case it might be needed to remove it first.
Or these two can be used to fix it afterwards:
Action #1:
Action type: Guess values
Source format: *%fbpm%
Guessing pattern: %fbpm%
Action #2:
Action type: Guess values
Source format: %fbpm%
Guessing pattern: *%fBPM%
Or last possibility is to add the field to the tag panel (Options > Tag panel) and edit the usrfields.ini as explained here:
/t/3738/1
Plenty of ways to fix it...
Be careful not to lose trailing spaces by changing careless or accidental while saving.
The .mta file underlies other design principles than the standard .ini file.
DD.20081016.1405.CEST