Change map shader at runtime

Is it possible? I mean I can change a models shader with ent_mtlset, but how do I change it for the map?

set different materials to blocks you want to change independently (originally they can have the same shader), and:
effect_load(mtl_flat ,"your.fx");
effect_load(mtl_shaded ,"your.fx");

Awesome! Thank you very much, it works like a charm