Tried your system. Did not work. I clicked on edit/modify. The pts/certificate is one button and checked off, of course, so I unchecked it thinking that would do it but I got some message saying that my room type was not available.

I decided to call in with explicit instructions to detach my certificate and the she asked m something about what I want to attach it to. She asked about cancelling and I said yes but emphasized about detaching certificate first. She did seem to know what I was talking about. She said it was done, to refresh my screen which I did but did not see anything. Wouldn't you know it, the call disconnects or she hung up on me. I called back mentioning that I had just called. Now the second rep was telling me that there was no certificate attached and that I used points although no points were deducted. Okay, so it seems she figured out that the previous rep did detach the certificate but did not re-post it. Now it's done. I knew this wouldn't be easy but just want to mention that if anyone is doing the same, call the Marriott Bonvoy rewards number with specific instructions to detach the certificate before doing a cancellation or just detach and cancel later yourself online.

The upside to all this is that the re-posted certificate is good for another full year.

BTW, there is no hold time when calling.