Hey everybody. I was wondering if this is possible, and if it is, why it isn't working.

When you right-click an application (.app), there is an option for "Show Package Contents." If you select that option, it opens up a "Contents" folder. Inside that folder, there are multiple folders, but one in particular, the "MacOS" folder, seems to be common to all applications. Inside this folder is usually a single Unix Executable File. If you run that, the application is basically run from the terminal, or at least that is what it seems like to me. Now, the question part:

Is it possible to replace that Unix Executable with another Unix Executable so that when the program is run (double clicked), it runs a different program?

Yup, i always do that when hacking the software. But if you are not experienced enough it can corrupt the other files in the software, rendering your software useless and can't be use any more. In serious case it can harm your computer.