A question for deleting classification from 'Class Structure for Part'.

Hi Aras community, 

Would you please help one thing?
In Part ItemType, I tried to delete 'Machine' from 'Class Structure for Part'.
We have couple of parts which have 'Machine' Classification.
So, I updated 'Machine' in Classification field in part table in database with 'new classification'.
And then, I tried to delete 'Machine' from 'Class Structure for Part'.
However, system shows the error message below:

Is there a way of deleting 'meta-data references' on this class?

Thank you so much for your help in advance. 

Regards,

Joseph