Changing non-member status to member of another unit

Recently a former member of the ward visited our ward and made a tithing donation. When entering the donation, I incorrectly entered the former member as a "non-member". I have gone to the donors list and attempted to change it to "member of another unit" by clicking on that button, but that doesn't work. Any suggestions would be greatly appreciated.

You have to create a member donor record and then merge the nonmember donor record into the member donor record.