Originally Posted by HarbourGent

If you're talking about skipping the economy HKG-BKK leg of your paid ticket and instead using an Avios redemption for that leg only, missing the paid leg would invalidate the rest of your ticket. If you haven't flown that segment that would be true even if you checked in for it (which would likely create problems itself if using the same travel documents to check in for the same flight on the redemption tickets). Cathay is not at all lenient re skipping segments as I have found in the past.