Changing geometry of of instance of a warddrobe item

If I hide and then delete some polygons on say a pair of jeans, then those polygons end up deleted from all instances of the jeans, even if I load a new pair from the content library. What can I do to make to make a copy of the clothing so that I don't affect the original piece? I tried "duplicate node" but the duplicate and the original were both affected when I deleted some polygons.
Comments
I did try and export it as an obj I think and reimport it but it lost all rigging and morphs.
Also when I save and reload a scene with polygons removed from one clothing item, they're magically reapplied. (I mean I prefer that to the polygons being forever missing from the wardrobe items, but there's got to be an easy way to save an altered piece of clothing including all rigging and textures and morphs?)
Hmm, even primitives are sometimes not loaded as they were when I saved the scene, i.e. without certain polygons.
After you deleted the polygons save the clothing item as a new asset : File -> Save As -> Figure Prop/Assets, give it a new file name (don't pick the old one since that will overwrite it) and in the menu popup change the Product name or Item name to something else by putting a number after it. That way it won't save over the info for the original item and you can load the original and the new version.
Thank you Mada!
I noticed saving things as a new asset doesn't help keeping invisible polygons hidden, only deleted ones. Just a heads-up for everyone. Should this also be reported as a bug, perhaps?
Select the polys you want to hide, create a new materials zone for them and make it invisible, save the scene or save the modified item as a subset! The polys should be invisible when you open the scene/load the subset.