No, simply changing the fan speed on your GTX 780Ti will not overclock your GPU, although it may allow you to run longer at a higher turbo bin.
Overclocking your GPU means that you are increasing the GPU and/or Memory speed beyond the limits set by the manufacturer.
This will generally increase performance, power consumption and heat production from your GPU.
It can also lead to instability or even lower performance if not properly done and tested.
If you are satisfied with your current performance and are uncomfortable with the risks, I would recommend keeping your system at stock.
You can always overclock in the future after you feel your performance levels are a bit to low and you have done a bit of indepth research on overclocking.