Thanks for the responses. According to this post from Fly with Moxie (and Google Translate), it appears that you would be able to preserve pre-devaluation pricing for the return segment of a roundtrip award ticket (if you had to change it after April 2021), but that's not necessarily the case for one-way award tickets. It appears that you might be able to keep a one-way award ticket at the old pricing if you are just rescheduling dates, but any reissuance of the ticket post-April would fall under the new award chart.

I wish Korean Air would postpone these changes altogether with COVID-19 still raging on throughout the world and its impending merger with Asiana!