if you check this site - https://www.routesonline.com/news/ta...55/korean-air/

You'll see that around 3-4 months before (Sept 2019) - they start announcing winter aircraft changes.

Example: I bought a ticket to Bangkok in August for Christmas. In September the time and aircraft were both adjusted (but I don't expect anymore change). So I'd expect one more change (or finalization if you will) in like January-ish.