When I put #include <ctype.h> at the top of my source file, the compiler finds it. If I add #include <reg688.h>, I get error "can't open file 'reg688.h'". If change it to read #include <philips\reg688.h>, it still can't find the file. And if I put the full path in ( <c:\keil\c51\inc\philips\reg688.h> ) it STILL CAN'T FIND the file. What is this stupid compiler looking for?
Don't worry about changing the project when you do the your job of extending the project.
You are expected to use source-code management, i.e. the changes you do will not destroy the original project. Instead, the source code manager will be able to restore either your version or the original version. And it will be able to show every difference between your new version and the original that the consultant left you with.
If you do not use source-code versioning, then it is time to stop all work and first think about how to work with source code. Any project that matters will require traceability, and that will require source-code management (and of course similar management of the requirements, test cases, bug tracker, ...)