Could I load the version 2.1 on my HW3 without any problems? I would run it in hosted mode with the added Rockbox. I don't know if someone here has already tried this and could tell me if it worked. Also, is there a video converter to the format MPEG that can actually run in the HiFi Walker? All the videos I've copied ended up in a restart/reboot of the device (it hanged and turned off and back on by itself).
What brand is your player and what brand firmware are you referring to? Each brandname has (frustratingly...) their own version numbers.
To my knowledge a hw3 device will not even accept a hw4 update file (they use different "device name" fields).
I actually have had success using both VLC and ffmpeg to encode videos for Rockbox - I had to mess with the settings for a while until it worked, see
https://www.rockbox.org/wiki/PluginMpegplayer.html for documentation.
I found this combination of settings to work when converting in VLC:
Encapsulation: MPEG-PS
Video Codec: MPEG-2, Bitrate=0, Frame Rate=0, Width=320, Height=180, Scale=(blank)
Audio Codec: MPEG Audio, Bitrate=128, Channels=2, Samplerate=44100
But of course there are no guarantees.
Give that a try maybe?