UpdateRecord currently cannot replace the root of a Record with the output of the unescapeJson RecordPath function.
While a child Record within a Record can be "moved" to the top-level, e.g. / => /child, this doesn't work if the RecordPath function is used, e.g. / => unescapeJson(/stringified_child).
Rather than replacing the Record content with the output of the unescapeJson function, a root field is added to the input Record (i.e. with the name root)) and the de-stringified content of the selected field(s) serialised in Map format.