How to swap the contents of 2 tag fields?
Example how to swap TITLE and ARTIST:
Create a new action in an existing action group or make a new action group.
Action type: Guess values
Source format: %title%~%artist%
Guessing pattern: %artist%~%title%
Note: The ~ sign is just used to separate the two fields, you could take any other sign that does not appear in your fields, e.g. ¢ or an extended unicode character like ∞ or ∑.