In my former life I did a lot of data migration, moving finance and other business information between old and new systems. Moving metadata between cataloguing systems isn't very much different and I've moved quite a number of photographers' metadata into iView (now Expression Media) using an Excel application as a bridge (from there it's an easy enough jump to Lightroom or Aperture).

I've now packaged this Excel spreadsheet up as a more generic tool that clients can use to run their own data migration. While it is for Windows users, there's an alternative route for Mac users. For more info, read this pdf file. According to the client who used the initial version to update 50000 items, “does the job beautifully“.