Barbaros,
You aren't mentioning which Cincom Smalltalk the existing code is written in - VisualWorks or Object Studio...? Or maybe even VSE?
Chances are most of your non-GUI code will be portable quite easily. There may be some Collection methods and other selectors that differ between the dialects, but that can be handled quite easily with tools like the rewrite tool which ships with the Refactoring Browser.
If your application is a GUI application, this is a completely different story. All Smalltalk dialects have their own GUI architecture which basically differs so much from all others that you will most likely end up rewriting GUI code by hand. If your old Smalltalk is VSE and the GUIs have been written in WindowBuilder, then you may have the best starting position: Instantiations offers a tool to convert VSE/WB GUIs to VA Smalltalk/WB GUIs (
https://www.instantiations.com/products/add-ons/windowbuilderpro-translation-toolkit.html).
They offer more or less full-automatic conversions between Smalltalk dialects and/or from Smalltalk to other technologies.
It would be interesting to learn more about your conversion project. Your question has been asked several times in the past and will most likely be asked a lot more in the future, so please share whatever you learn on your journey...
Joachim