I think arena matches are not frequent enough to make a whole new arena, which would most likely be less interesting than the one I made years ago. Creating a duplicate just to change the lava doesn't look like a good idea, either... The secondary jails had some complex triggering and I'm afraid of breaking that if I make a new one. Feel free to try by yourself, but you may end up breaking something!
I think turning the surfaces into movers (with the lavavolume attached to them) would be an easy way to do it, it's changing the lighting that's the hard part...
BTW, I have the original LoadingScreen file here. Let me know when you're done with the JB version changes and I'll update the picture... or I could send you the .psd file if you want to do it instead...
Edit:
Just one thing: There seem to be many BSP surfaces that have a needlessly small lightmap resolution.
By 'small', you mean high or low resolution? There may be some stuff in the middle rooms with hi-res lightmaps to make the ramps create shadows, but I think I 'lowered' (to 32 or 64) everything else to reduce the filesize...
Edit2:
It seems that dynamic light effects can be done with Projectors, any idea of how they work? Maybe the lights near the lava could be changed to white, using a red/green projector to color the walls.
Projectors can be turned off in the game settings, though...
Edit3:
Made a quick test with TriggerLights and they seem to work. Doing this in one or two lights in the arena/jail may be enough, although I don't know if it would be too performance-heavy...