I have a noname MT6572 tablet with Android 4.2.2, rooted, and I've figured out the trick: I've changed mount point directory names in the fstab.
The file is:
/system/etc/vold.fstab, original version:
dev_mount sdcard /storage/sdcard0 emmc@fat /devices/platform/goldfish_mmc.0 /devices/platform/mtk-msdc.0/mmc_host
dev_mount sdcard2 /storage/sdcard1 auto /devices/platform/goldfish_mmc.1 /devices/platform/mtk-msdc.1/mmc_host
The modified version:
dev_mount sdcard /storage/sdcard1 emmc@fat /devices/platform/goldfish_mmc.0 /devices/platform/mtk-msdc.0/mmc_host
dev_mount sdcard2 /storage/sdcard0 auto /devices/platform/goldfish_mmc.1 /devices/platform/mtk-msdc.1/mmc_host
As you can see, I've only swapped
/storage/sdcard0, then rebooted.
I'm not 100% sure, probably Kitkat has different startup mechanism.
Caution: make backup! Move all your file editing applications to internal storage, so if something goes wrong, and you found yourself without any storage mounted, you can restore or repair it with apps launched from internal storage.