Is there a global way to merge an iPhone and an iPad app into a universal app?-Collection of common programming errors

I wrote an iPhone app. Then, I changed the interface a fair bit, added higher res images, and made an iPad version of it. In a perfect world, I would like for anyone who buys the iPad version to get the iPhone version for free (though not conversely). Since Apple doesn’t seem to have a way to do that (right??), my next favorite solution is to make the iPad version include the iPhone version somehow.

I know that I could just convert the iPad version to “universal”, but since the face of the app is so different, that seems like a real pain. I have many view controllers and they are almost completely different between the two versions. Some of the methods are the same, but only about 30%. Is there an easy solution along the lines of this:

• Check if device is iPhone or iPad

  • If iPhone, then use one group of files

  • If iPad, then use a different group of files

Thanks in advance!