This is the way I do it (right or wrong):
Left-click the component to highlight it.
Right-click on the highlighted component and select Edit Part.
Edit the power/ground pins and give them a length and make them visible.
Save the part (update current only or update all in the design)
Then connect to the proper power nets.
I guess, an option would be to change the pin name to the new power net (and keep it zero-length and hidden) but for clarity, I prefer to connect them to a net that is visible. My schematics assume VCC and GND for hidden power pins and visible connections for any other.