LR4: Delete "master" rather than virtual copy
After importing a library from Mr. Laptop, I was left with a few duplicates that presumably resulted from forgetting to raze a memory card on one occasion. I haven't previously used virtual copies, and would rather have a single version of each image for now. However, the virtual copies are the ones that have been edited, and I want to keep those edits. So in each case the "master copy" has to go, without erasing the RAW file from disk. Erasing a virtual copy is straightforward, but how do I erase the master and make the virtual one the master instead?