After some experimenting I found that this works:
vector rot_old = llRot2Euler( llGetRot() ) * RAD_TO_DEG; // get the original rotation in Euler format
vector rot_new = rot_old + <0,0,1>; // calculate new rotation, turned 90 DEG over Z-axis
llSetLinkPrimitiveParamsFast( LINK_THIS, [ PRIM_ROTATION, <1,0,0,1> * llEuler2Rot( rot_new * DEG_TO_RAD ) ] ); // apply new rotation
good luck!,
RJ