Generally they are relatively flexible... the highest reissue fee I have ever seen, on a very discounted ex-Europe F ticket was 250 EUR. An F ticket bought today VCE-SYD-VCE would cost ~5k EUR if A inventory was available, and has no reissue fee.

It sounds like there is no A inventory (the lower of the two F buckets) available and you are being charged for some kind of re-fare... if you tell us the dates you wish to fly on we can look at the availability and perhaps provide you with options.