Watterott's design files are for Eagle, while I normally work in KiCAD. KiCAD has perfectly workable importers for Eagle schematics and PCBs, with the minor quirk that you have to start eeschema and pcbnew directly (instead of via the main KiCAD application) for the import option to appear. However, updating the PCB from the schematic failed. I didn't spend much time investigating this, as I planned to redo the routing anyway.

The resulting board still looked like an Eagle schematic and used Eagle symbols, which isn't great. Luckily, this was easy enough to manually fix, though a bit tedious. One annoying issue was that most symbols from KiCAD libraries were oriented 90° off from their Eagle counterparts.

The original schematic contains a symbol for a solder jumper. I left this untouched (to the annoyance of the ERC) as I plan to remove that jumper in my final schematic anyway.