I'm not sure how CX can be right?

On the one hand their (CX) rules are not in the public domain. The Oneworld rules appear to allow change of 1st coupon according to rule 16 without charge.

The only reference to changing the date of 1st intercontinental sector is in, the rather oddly numbered "0" of the technical rule sheet which states:

[I]The date of the first intercontinental sector determines the applicable seasonal fare for the entire journey.
Seasonality will be determined by the country of commencement of travel. If the outbound intercontinental
sector is open dated charge the peak season fare./I]

Once again according to Oneworld's website, seasonality only applies to Economy fares.

In my case, the fare is still the same (as I had ticket reissued before 1 May), only the discount no longer applies.

So I guess one is faced with 2 sets of rules that apparently contradict each other - one is in the public domain and the other is not, and the one that seems to apply is the unknown one.

Furthermore, when the ticket was issued, there were absolutely no conditions that you would expect where there are restrictions on amendments etc.

Above all, it certainly is not within the spirit that Oneworld announced the 10% discount.