I suppose nobody here knows the answer to that.
If you're willing to be a guinea pig for us, you could try wiping out everything in /mnt/ext1 via USB just before copying over the new firmware file. I wouldn't worry about wiping out any SD card you use; there is no system stuff that goes there. Dictionaries and TTS files are in a hidden partition, so you don't have to worry about losing those.
I'm not sure this won't leave your machine in a hung state once you unplug the USB cable, so I'd make sure the new firmware was copied over before unplugging the cable.
Again, I stress that I've never done anything like this, so my suggestion could be totally wrong and cause you much grief. Please keep that in mind!