thanks. Your response further confirms to me/reasures that I will never get charged the price differential by throwing away the last segment.

Rational: if they are offering to move dates, it means they don't see the ticket restrictions straight away on the call. They only check ticket condition when I indeed ask them to move at which point they realize it's a problem and potentially challenge me or collect fee difference.

So as long as I say "I will rebook when my dates are firm, I don't need booking for now" which isn't a lie technically,, they just think I will rebook at one stage. And the the ticket which was married etc etc will just lapse and no one knows. They make their own assumption so I'm not liable for this.