Is it possible to transfer or convert Marriott Rewards points to American Express Reward points?

I know that it is possible to convert American Express Reward points to American Express Reward points but I don't see how to do the reverse. I almost want to say that I have done that many, many years ago but I don't see any way of doing so now. Am I just imagining things or did Bonvoy and/or American Express change their policies and/or am I just overlooking how to do it?