UPDATED: This Tag That Tag v4.3
This Tag That Tag will assist with swapping, copying, and appending data between two user-chosen tags in selected tracks or tracks in the selected playlist:
Swap - swap data between tags, ex: ARTIST<->COMPOSER
Copy - copy data from one to another tag, ex: ARTIST->COMPOSER ARTIST
Append - append data from one tag to the end of another, ex: ARTIST->COMPOSER - ARTIST
Prepend - prepend data from one tag to the beginning of another, ex: ARTIST->ARTIST - COMPOSER
This lateset version adds "Date Added" and "Release Date" to the "This" tags. Like the number properties already available, date properties cannot Swap with the text properties available as "That" tags because date properties can't accept text. The dates can be coerced to text in this format: YYYY-MM-DD HH:MM:SS so that they can be sorted properly.
This Tag That Tag is free to try for 10 days, $1.99 thereafter, free for registered users. More info and download is available here.