This is an issue for mappers who use prebaked lighting.

That may very well happen, not for the reason you suggested, but rather for performance reasons.

And then that default would be changed by user, or mappers?

I agree with Smoth, rather require a tag on new/updated maps to make use of a feature than make some old maps worse.

viewtopic.php?f=12&t=25504&hilit=+dynamic+sun+
Ill make mega disco sun

On shading it won't really matter much, unless your shading was very hard baked into it.

Perhaps best option is on/off toggle in the map (in which case default would be off)?

sonIamdisapoint.jpg.

I downloaded test version and indeed get very low FPS. 10 fps in situations where I got 50 before.
/advmapshading 0 gives me back the old fps. (it still seems a bit lower but maybe thats some different settings)
But then I get no shadows at all? Is it somehow possible to have normal, non-dynamic shadows?

DynamicSun=0

Other settings to try for more fps are
MaxDynamicMapLights=0
MaxDynamicModelLights=0

Dynamic sun will absolutely not kill that much FPS, unless your graphics driver is broken. My FPS drops from 60 to maybe 58.

ok, will test some settings and see whats problem then.

To avoid future angst, please allow for a modoption to turn it on by default for that particular game.