I am also a charter Marriott Ambassador member. A few months ago, my ambassador emailed me to say that I would maintain status through January 2019 but would need to qualify after that.
If I look at status in the app, it's my sense that they've done something to code those of us who are legacy MR Ambassador members. For example, mine says I have ambassador status through January 2019 even though I'm well over the 100 nights/20k. Newly qualified folks show they have ambassador status through January 2020..
Bottom line, I think you need 100/20k to requalify.