Half the time QF's system itself isn't even able to ticket bookings within that tight timeframe. History suggests the DSC are awarded based on PNR creation, and I don't believe it will be any different this time. There is certainly nothing within the T&C which indicate this to be the case. Indeed, many premium cabin fares have TTLs of up to 60 days after booking.
"If you change or cancel your initial qualifying booking and/or travel is completed outside the promotional period, this bonus offer will not apply."
My interpretation of the clause regarding flight changes is that ONLY PNR creation date determines the awarding of DSC. This particular clause suggests that changes to the booking are allowed, and only in cases where a segment has been moved outside of the "Fly by..." period would it become ineligible. Reading between the lines, this has to mean PNR is the sole measure of eligibility, as once a voluntary date change is made a ticket has to be re-issued/re-validated and that would actually cause the whole booking to become ineligible.
Just my intepretation, but I am pretty confident.