Consider the case where the same logical control in your application is implemented using different classes on different GUIs.
If the kinds of actions you can perform against the object classes are similar, and if Silk Test Classic uses the same method names for the actions, then you do not have a portability problem to address.
For example, the methods for the RadioList and PopupList classes have identical names, because the actions being performed by the methods are similar. Therefore, if a control in your application is a popup list on one GUI and a radio list on another, your scripts are already portable.
If the two object classes do not have similar methods, or if the methods have different names, then you need to port your scripts.