Here is what works for me:
- Open the library in Explorer
- Select all files
- Copy all files
- Paste the files locally
As each file is pasted locally, the properties are updated in SharePoint - you don't have to copy them back to the library or anything - simply copy them out of the library. Try it with one or two to make sure it works, and yeah, with 15,000 files it will take a while, but it should get you squared away.