In 10.1 and all previous editions when a file is opened that is flagged as
READ-ONLY and then the file’s property is changed you can edit the open
file. In 10.6 you need to close and reload it.
However 10.6 does know when I open file has been changed by another app and it
asks you correctly if you want to reload.
It is just that the property of read-only is not updated when it becomes
updateable. The property is updated when it becomes READ-ONLY.