TxToJSON() and TxToUniv() are only to be called when we want to decode the transaction (i.e. its details) into JSON. If TxVerbosity is SHOW_TXID, the function should not have been (and currently is not) called in the first place.
There is no behaviour change, current logic simply assumes anything less than TxVerbosity::SHOW_DETAILS_AND_PREVOUT equals TxVerbosity::SHOW_DETAILS. With this change, the assumptions and intent become more explicit.