skimask's keyword is "rate". That won't change with voltage only.

Given the voltage and the load on the capacitance of the LED the rate will always be the same.

Only by changing the load presented to the LED will change the rate. And now it came to me an idea. Why not put a second pin of a PIC in parallel to increase the load when you want and then set it at tristate again? May be and a third one?

Ioannis