Changing Default Animation with Lua

Is it possible to write a script at the beginnig of a scene to change the Default Animation of different objects?

Hmm. both luadocs page & the wiki data structure page say it isn't scriptable. You could just leave the default animation empty & instead create a called by other action block that you  can call in the at begin of scene action block. You just need to add some if condition/value type queries into the called by other action block to determine which animation to play with the play animation action part.

Thank you for another wise advise.

Just because something says it isn't scriptable, doesn't mean that it isn't. It usually just means that the change won't be saved in the save game data or might require a scene/animation restart, etc.