Yes according to the "how to amend a return" section on this forum.
I tried it back in August, pretty soon after it was announced that we could, I checked all the correct boxes, error check went through with flying colors, the return was rejected a day or so later. I was amending MFS to MFJ which was allowed according to the instructions but that was the reason for the rejection.
I was prepared to paper file until I notice the announcement that we could efile amend returns we did not prepare. So off in the mail it went