In order to change power, what else must be changed?

In order to change power, current or voltage should also be changed. Voltage is an electromotive force, and also the quantitative expression that shows the potential difference of the two points charged in an electrical field.  So, before power will take place, it would always be best to change also the voltage.

In order to change electrical power, current or voltage or both current and voltage should be changed.

for changing mechanical power, work or time or both should be change

Explanation:

Electrical power

We know that electrical power is given by following equation

therefore, power depends on current and voltage and to change power, current or voltage or both current and voltage should be changed

Mechanical power

also mechanical power is given as

Power=

therefore for changing mechanical power work and time should be change

the answer would be   dignified - apex

thermal energy is the answer