Version 1.x owners should uninstall version 1.x before installing version 2.0. The Help File and the Path variables can be left as is (assuming you are going to put version 2.0 in the same place). Remove all packages from Delphi and close all projects in Delphi. Only then, should you build the IECS version 2.0.
Version 2.0 represents a major upgrade and many things have changed. In general, these changes are "underneath" and shouldn't affect your code. However, many optimizations have been made which can affect your programs. Version 2.0 changed some interfaces and event signatures for faster execution (mainly, the const keyword was put in method calls to avoid costly Add/Release calls).
If you just used the IECS to create expert system files and did not write any code, you should be unaffected. The only changes that might affect you are event changes. Many event signatures were changed to pass interfaces by const. If you get a compilation error with your events, check if they pass interfaces and add the const keyword.