See [forums.reprap.org]
Then all you need to do is change the serial port in your firmware.
NB to upload new firmware you will still need to use serial0, unless you also update the boot loader code.
NB you do know you can get at TX0/RX0 on a ramps, bypassing the usb to serial converters? see AUX-1 port, D0/D1 is the same pins as RX0/TX0
This is how tft28 boards talk to ramps.
Then all you need to do is change the serial port in your firmware.
NB to upload new firmware you will still need to use serial0, unless you also update the boot loader code.
NB you do know you can get at TX0/RX0 on a ramps, bypassing the usb to serial converters? see AUX-1 port, D0/D1 is the same pins as RX0/TX0
This is how tft28 boards talk to ramps.