UDT expressions can be used in an upsert form of UPDATE statement in any way that is consistent with the general support of UDTs by INSERT and UPDATE and with existing semantics for UPSERT. See Inserting into UDT Columns and Updating Distinct UDT Columns.
The UPDATE statement used in the upsert operation can also use a mutator SET clause if it updates a structured UDT column. See Updating Structured UDTs Using a Mutator SET Clause.