changing osc on the fly

[tenaja]

I may be wrong on this but...

Power consumption is relative to voltage. Clock speed is relative to voltage.
Faster the clock, higher the 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.

???

I run this PIC at 3.3 volts and 48MHz but I have not measured the amps...

That is all true... but you will save even more power by putting the pic to sleep. A a high speed pic that is sleeping most of the time will draw less power than a 4mhz pic awake all the time.