it might be possible to replace it with a pic that has the same serial and usb connection capabilities and one with enough memory (or if you add some memory) off hand i dont know of one that does, you could look at the digikey catalog, they usually list the features in a comparison format for pics.
Microchip samples that device. Also, maybe a 18F4455. Same device - less memory. It may still have enough. Perhaps worth a try. Microchip also samples that device.