Is this just dates change?
 

My parents have OWE tickets for HKG / SEL / HKG followed by HKG / BKK / HKG. What if they swap and fly first to BKK and later to SEL? Does the US$ 75 penalty apply?

I would say yes. Even though the cities are the same, the order and hence the itinerary has changed. Nobody likes to pay a fee for changes, but considering that it costs $100 for a domestic ticket change, $75 seems reasonable since these tickets are thousands of dollars anyway.

CX in HKG will have to re-issue the ticket if your parents want to go to BKK first. There will be the $75 fee exchanged to HKD$.