However, as has been pointed out elsewhere there is a way you can fairly easily copy your existing captions to the right field.
- Get and install exiftool which is a command line tool for manipulating photo metadata
- Copy the IPTC Caption-Abstract metadata to the exif Image Description:
exiftool "-exif:ImageDescription<IPTC:Caption-Abstract" -r -P --progress <targetdir>
Just replace <targetdir> with the directory containing all your photos and this command will recursively process all your photos (reasonably quickly too!)
- Import you photos into either iPhoto or Aperture
Even so, it is probably a good idea to make a backup of your valuable photos before running it on your whole library.