Broken Serial port

I bought a Chinese clone from Bangood.
The item comes with the GPS Serial port broken.

I am trying to remap the serial device used internally to anothe unused serial port.

I found some post sharing sucess stories about this:

but I am wondering if there is a easy way without modifing the firmware:

As everyone knows the init rcS script call for a script in the SDCard:

47 # Look for an init script on the microSD card.
48 # Disable autostart if the script found.
49 #
50 set FRC /fs/microsd/etc/rc.txt
51 if [ -f $FRC ]
52 then
53 echo “INFO [init] Executing script: $FRC”
54 sh $FRC
55 set MODE custom
56 fi

My question is: Is it posible to rename de TTY device at the boot time with a config file in the SD Card like this?

mv /dev/ttyS3 /dev/ttyS20
mv /dev/ttyS4 /dev/ttyS3
mv /dev/ttyS20 /dev/ttyS4

Hey guys, good news here. I had problems with the above procedure, just forget it. I`ve installed firmware 3.4.3 and disabled serial 3 and 5, set serial 4 as GPS option 5 baud 38. And believe me it worked!!! I am flying it as normal. POS HOLD LOITTER, RTL. everything works!!!
Please whoever need it, just try. Do not trash your old good pixhawk. I was almost doing it… Here you have a pic of my pix indoor.
It just works!!!