How can I port apps from OS X 10.8 Mountain Lion to OS X 10.7 Lion? In Mountain Lion, many of the day-to-day apps like Preview, TextEdit have been upgraded with some very useful features. (For instance, you can now insert a page into a PDF document using your scanner in Preview - see here, and pinch the trackpad for zooming in on to a document - see here). However, as far as I understand, these apps are free, and features like these are features of the app themselves, not the OS.

Is there anyway in which I can replace/upgrade individual apps in Lion, without having to install it as a whole (preferably as a dual-boot config!) ?
No.  Apple does not distribute these apps individually.
Even if you copy them from one Mountain Lion system to a Lion machine, they are very likely to not work as they are relying on system level framework (or functions in these frameworks) that won't be available in OS X before ML