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is there an event that does this: only if the user selects a different value from the combo(different from the previous one)something happens ....i mean instead of onedit ..... onchange ?

There is no special event for such case, but you can use onEditCell, just with additional check

grid.attachEvent("onEditCell",function(stage,id,ind,oldv,newv){
    if (stage==2 && oldv!=newv)
       do_something(); // do something when new cell value not equal to old value.
    return true;
})