Can I change VCC pin to 3.3V on a 5V Arduino Pro Micro? I'm using a 5V Arduino Pro Micro (schematic).

For my project, I'm using the VCC pin to output power to an external device.  I need VCC to output 3.3V to the external device.  Can I open jumper SJ1 to reduce the voltage of the VCC pin without affecting the operation of the other parts of the unit?
No. If you change the voltage to 3,3 Volt you also have to change the crystal to 8MHz and reprogram the ATmega32u4 to meet the new frequency.