it may be that the public part of a scratchpad is actually housed in the document table and so conversion isn't even a problem).
That is precisely the situation; so converting a note to a scratchpad would be trivial.
However, it does seem like making last hour of cb a document, rather than a scratchpad, makes some sense.
I don't see any features of scratchpad which would bring any benefit in this case.
And converting a note to a document is dead trivial: simply change its type from 11 to 3.
(Yeah, we'd orphan a row in the note table, but that's probably not worth worrying about one time.)
Between the mind which plans and the hands which build, there must be a mediator... and this mediator must be the heart.