I tried asking this on Quora, but it was never answered. Figured I'd try my luck here.
I won’t be able to finish the course before 31 December 2019. What will happen?
After 31 December 2019, the course will be updated to a 2020 version. Your progress from 2019 will be carried forward to the 2020 version. After 31 December 2019, you must submit the 2020 version’s problem sets. See the chart on the courses’s home page for instructions starting in January 2020.