An inadvertent designation may be changed if the change is done or attempted without pause for thought.
Since you attempted to change it immediately all the TD has to do is to decide whether you mean the diamond all along and said the wrong thing, in which case it may be changed, or whether you changed your mind in an instant, in which case it may not be changed.
From what you have said I would not be too surprised if the TD decided it was a change of mind and did not allow the change.