It's really simple, you need to set your filemanger to mount system as root. Then navigate to /etc/vold.fstab open as text, go down to dev_mount. Change sdcard /mnt/sdcard to extsd /mnt/extsd. Then the next dev_mount change extsd /mnt/extsd to sdcard /mnt/sdcard.
Only switch those words not entire line.
Then reboot, you are just changing mount points.