Sorry, but that is basically incorrect. (Unless in the unlikely event weeks or months down the track that you find a new base fare that is exactly the same as that originally booked.)
Say you have a booking MEL-SYD one-way for $155 all up. The actual fare in Q class is $129.52 with Charges/Taxes at $25.48.
You then need to cancel it. One now has a voucher "valued" at $155. This voucher can then be used to help pay for travel where the fare component is at least $129.52.
It can't be used for an O class fare MEL-SYD at $99.52 since that is less than the original fare.
It can be used for a V class fare of $136.79 (with $32.21 in taxes making it $169.00).
When used for that V class fare, the additional payment is the fare difference + the change fee.
+$169 New Fare
-$155 Old Fare
+ $60 Change Fee
-----
+$ 74 Additional collect
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The reason Qantas do this is to stop people buying fares then, after seeing a cheaper fare beome available using the credit instead. FWiW, when red e-deals were originally introduced they were unchangeable - one flew as booked or the fare was forfeited.