Deleting inventory that has been invoiced to a customer?

Hi there, 

My company has several Inventory Items that we would like to remove from our system for various reasons. Our supplier has discontinued them, upgraded to a new version, etc. 
However many of these inventory items have been sold to customers in the past, and are on Client Invoices. 
Has anyone encountered this, and what did you do? 

Thank you! 

Abby (Assistant to Mitchell)