The Arduino IDE uses C++ . Briefly looking at the MPLAB IDE I have seen mention of 'C' compilers but not C++. If you have a large amount of code already written then either you find a C++ compiler for the PIC or you will have to rewrite your code into C ( this could be a big or small task depending on whether yourcode really makes use of C++ features or not).
I guess your existing code also uses libraries. Those libraries may not be available to you, so you will either have to develop compatible libraries or rewrite your code to use alternatives available on the PIC.
Is your product already being sold? It seems strange to make such a big change so late in the game.