drivers - Changing nvidia graphics card after installing 16.04

I have installed Ubuntu 16.04 and at present I am using the graphics card: GeForce 8400GS. I would like to remove this card and instead replace it with GTX GeForce 980 Ti which I have.

1. If I keep the current driver (for GeForce 8400GS), replace the
   graphics card then ubuntu won't boot up.


2. I can't install the new driver (driver for GTX GeForce 980 Ti)
   without removing the current card.

What is the recommended way to changing the cards and the respective graphic drivers?

Thanks for your suggestions in advance.