Version 1.x and 2.0x owners should uninstall the previous version before installing version 2.5. The Help File can be left as is (assuming you are going to put version 2.5 in the same place). Remove all packages from Delphi and close all projects in Delphi. Only then, should you build the IECS version 2.5.
Version 2.5 represents quite a few changes in the engine "underneath" to make it compatible with Win32 and VCL.NET. As much as possible, source code compatibility with v2.0 Win32 was kept, but some changes were unavoidable. The most important changes were the introduction of a Common.hlpRSConstants.pas file, which conditionally defines some simple types, such as TRSPointerType (a Pointer in Win32 but a TObject in .NET) and TRSStringPointerType (a PChar in Win32 but a String in .NET). All methods were changed to use these new types. Also, your "uses" clauses may have to change as some types have been moved around to the new constants file.