If you're using a device like the 16F917, I think you can change the speed on the fly by writing to the OSCCON register. However, there is a catch. How will you inform PBP about the speed change? The define OSC 8 will set the speeds for most of their functions like pause and I2C timings. I dont know if they read the OSCCON to decide or just use the define. So, I guess you'll have to tread with caution