An SolidWorks assembly (machine.sldasm) contains a pneumatic cylinder (pneumatic.sldasm) which is an assembly of a piston and the cylinder tube.  It is marked as flexible in the properties, so I can move the piston of the cylinder in the assembly (machine.sldasm), but then I change the original cylinder file too.

How can I play with the movable parts without changing the embedded original files? 

Best regards