I've pre-emptively migrated a couple projects and found that loading the old corpus files wherever you already had them and then `Add`ing them as whatever new appropriate type was the easiest way. The inclusion of types necessitates at least a minor migration. I did not find any official documentation on the format, though it's trivial to read, e.g.:
go test fuzz v1
string("\xff0")
Overall while the API (and of course tooling) is a huge step forward, corpus management feels like a small step backwards compared to go-fuzz - I didn't find a way to pull non-crashers into an in-repo corpus other than manually copying them out of my cache directory. And one-file-per-case still blows up a lot of repo management tools.