AutoCAD Inventor :: Discard Model Changes

When I'm working in an assembly, and I open up a model and make some changes, sometimes I decide I don't want to keep those changes anymore, but I've made to many and I max out my undo button before I can get rid of all the changes.  

If I try to just close th model, it just tells me that it is still open in the context of another part (the assembly) and that I can save it now, or with the assembly.  It doesn't give me an option to discard my changes.  

What I end up having to do is select "No" on that option and close the part, then save the assembly, and go through the list of parts it is saving and tell it not to save the part in question, and close the assembly, and repeat for all assemblies it is contained within until the entire model is closed, then open everything back up again.

Any easier way to discard all of my changes since opening a part?  I have a strange feeling though that there is an issue with that, since any change you make on the model is updated in the assembly in real time.

I should add, I want to increase my undo file size, which is currently sitting at 1GB, but have one question: is the undo file stored in RAM or on the hard drive?  How much can I safely increase this without running into any performance issues?