It should absolutely fail: having a mixed case is incoherent: a user is sending something that is neither an old form message (in which all fields are old style `{"name": "example"}`), nor a new form (in which fields are strings), but some admixture of the two.

That is a format that would never have worked, and it would be very strange to me to think of supporting conform as defining a range of new message types that were not supported, but are now.

Basically, I see this as a migration over the whole message, not just the properties considered individually.  Ultimately, I don't mind if we support through an accident of the code more than we promise (eg: happen to support mixing forms between task and broker), but worry about trying to support mixing forms in a single property.)