When it comes down to whether or not alternates refer to the planning stage or the 'all stage' scenario, remember a forecast is only required to be obtained prior to flight (not during flight) covering the duration of the flight up to 60mins after. If you obtain a forecast and it's clear then at some stage after departure it is updated to below alternate minima but you never hear about it, does that mean that you're flying illegally without knowing about it? If alternates were required to be replanned in flight, wouldn't they also have made it a requirement to obtain updated forecasts in flight at X minute intervals to enable you to do this? To reiterate, you are only legally required to obtain a forecast prior to flight, and as alternate planning depends on having obtained a forecast...
CASA use the term 'in the spirit' of the regulation quite regularly when having these debates. Whilst I don't particularly agree with that phrase, I'll bet they meant it something along those lines when you tie in flight planning requirements (Jepp AU-601) with alternate planning requirements.
Having said that, I do agree that it doesn't mention explicitly in the alternate section that it is purely a pre flight consideration so either way, it requires some clarification.