I can't speak for Cathay specifically, but on most airlines, even if you have a fare with no change fee, you would still need to pay any fare difference between your original fare and whatever the amount is when you make the change on the return.

As an example, a few years ago I (my company's customer, actually) paid $1800 for a round trip from Orlando to Oakland, CA on United, in coach. The fare was fully refundable. I ended up finishing my work a couple of days early, and called UA to see about moving my return up two days. The only fare available was $145 more, and would require me to buy up that amount (still in coach).