As mentioned already, we'd also need some sort of version identifier, and we'd expect the load_statistics() functions to be able to transform the data if the old version used a different representation. I agree with the idea that an explicit representation of the source table attribute's type would be wise, too.
There is a version identifier currently (its own column not embedded in the JSON), but I discovered that I was able to put the burden on the export queries to spackle-over the changes in the table structures over time. Still, I knew that we'd need the version number in there eventually.