At the moment it seems that, when somebody reviews a line, adding an inline comment in Differential, then that person cannot change the Done/Not Done flag again. Note that, the author of the patch, instead, can.
This seems a bit counter-intuitive to me, since the author of the patch usually has lower-privileges than a reviewer, so it's strange that the patcher can change stuff made by the reviewer, but the reviewer cannot update their Done/Not Done flags.