What to do about switching from cymbalta to generic duloxetine?

Just switch! If you have been prescribed the generic form just switch It is unlikely to be very different.
Your choice. When choosing whether your want generic or brand, it is a choice you can make with your doctor. If you see a difference in efficiency, when informing your doctor he could prescribe brand, or generic only, depending on which one works better for you. Insurance might increase the amount you pay for brand versus generic.