It's embarrassing how easy this was; it just didn't occur to me to try it last night.
All I had to do was plug in the 950 and then start the Kobo desktop app. It loaded, and showed "Device connected: eReader" in the library, and asked me if I wanted to Eject the device or Sync it. I clicked "sync" and the books are now on the 950.
Stupidly simple.