A nice confirmed "benefit", but a few things to consider.
Firstly, it's "same day" - does this mean the same day in the location you booked it, or basically as compared to the timestamp in Sydney? For example, if you booked a flight in Singapore going to Sydney at 8pm local Singapore time, then you need to make a change, do you have 2 hours to call Qantas or 4 hours?
I assume "correcting the name" could mean anyone else. After all, it could happen, e.g. Dad booked a ticket for Mum to go see her friends, but got too used to clicking ahead on his account and submitting his details instead of the wife. He calls Qantas and embarrassingly explains that it's supposed to be her going out with her friends for a weekend, not him (oh really?

).
Swapping gateways might just give someone a shock in price difference! Especially the example (SYD-LHR rather than LHR-SYD). I assume that if the fare is lower, it will be both (a) acceptable and (b) a refund of the difference will be given (if the payment instrument has been charged at this time).
If you change fare type, I take it that restrictions re: changes etc. that are normally associated with changing fare types will not apply. For example, if one would normally change a Red e-Deal to a Flexi Saver, it doesn't mean that future changes of that changed ticket will be free of service fees. In this case with the same day guarantee, if one booked a Red e-Deal but intended to book a Flexi Saver, the "changed" fare will be such that your Flexi Saver will be as if you bought it from the outset as that.
Basically, it seems that the same day guarantee treats every ticket as a fully changeable one until midnight. The only thing you can't do, it seems, is, with the exception of a few flexible ticket types, get a refund, i.e. it doesn't seem that if you book a ticket in error that you can completely cancel it (unless the fare type allows full refund); same day guarantee will let you change anything else about the ticket (e.g. shift it to another day, "give it" to someone else, etc.), but you are still committed to buying something.
Finally, don't drink and book. You probably won't realise your mistake until you recover the next day, by which time it's too late and even this generous benefit can't save you

:mrgreen: