You won't need a switch.
The EMS-D120 input is a completely different serial port than the PC update port, so there's no conflict there.
The EFIS-D100 serial input is the same one used by the PC for updates, but now that our products no longer need their PC serial ports connected together to talk to each other (that communication is now handled exclusively over the DSAB bus), all you would need to do to hook the SL30 up would be to hook the single SL30 serial output wire to the correct pin on a fresh 9 pin connector. Then just hook that up to the EFIS-D100's 9 pin connector. When you need to update the firmware on the EFIS-D100, just pull off the SL30, and hook up the PC.