I do realise that this question most likely referred to Australian rules, however, it might be interesting to note that in NZ the rules include very clearly that, "at the time of submitting the flight plan", if the requirements aren't met, then an alternate is required.
Another prime example of how poorly written the Australian rules are. It seems with every law related question there is a near 50/50 split on the interpretation of the rule, and that is amongst the few pilots that can find the rule to offer their interpretation!!
If the TAF was changed en-route to BKN below the alternate minima, but well above the landing minima and with a METAR indicating good conditions also, then I've gotta say I would carry on, in most circumstances.
I can just imagine someone declaring a PAN PAN (or even diverting as some have suggested!!) just beyond the IAF for a landing in SKC conditions because the TAF has just been amended to include BKN cloud starting in 25 mins time 900ft above the landing minima but 100ft below the alternate minima!!
I think the 'spirit of the law' interpretation would certainly mirror the NZ law. I am sure CASA would have at least mentioned (in any of the AIP, CAAPs, CAOs, or CARs) en-route changes to TAFs and subsequent requirements if that was not the case.