It is not possible to change class on a part from class: P to class: X or V. 
A part with Class:P is defined as a product that is to be in stock and included in the product/stock accounts. 
A part with Class:V/X is in RamBase considered as a part that is only used for expenses - and is not included in the product/stock accounts. 
It is possible to use the F12 option: Make copy of Part and register a new part with the Class V/X.