changing osc on the fly

[rmteo]

....Clock speed is relative to voltage.

Not quite. There are minimum voltages required to run at specific frequencies but there is not a direct relationship between the 2.

....Power consumption is relative to voltage.

But with this type of PIC it will run at 2 volts with a 4MHz external.
So, using the PLL the PIC can "virtually" run at 48MHz on 2 volts and not consume extra power.

You cannot run it at 48Mhz with a 2V supply. With a PIC18F4550 supply current can range from 0.8mA (4MHz INTRC, 2.0V) to as high as 50mA (48MHz EC, 5.0V), a range of more than 62:1 - see section 28.2 DC Characteristics: Power-Down and Supply Current of the data sheet.

Using one of the Power Managed (idle/sleep) modes is the proper way to go. See Section 3.0.