I have used assembly for PIC16, but I am migrating to PIC18. I know some basic C, but I need to know PIC-specific C programming. For example, I don't know how interrupts work in C for PIC18.
If you are a good C programmer and all your problem is about PIC specific programming then all what you need is to see a full code example , you do not have to bother yourself learning again all aspects of C programming.
To see various c code examples refer to microchip site.