If you choose to upgrade MPLAB, don't delete your old version of MPLAB, but rather try a fresh install in a different directory. Many people that try MPLAB X go back in short order. YMMV.

As to my comment - I'm simply suggesting that perhaps Version 3 is less tolerant than 2.6 was of trying to define something that does not exist - certainly that's what the error says to me. Those defines are not sacred. You can add and remove them as you see fit. Don't worry about even editing "system files" - feel free to experiment. Just keep a pristine copy of the original files so you can go back if you mess up, and then experiment to your heart's content. I don't think it is possible to execute a "halt and catch fire" instruction on modern devices. .