Unable to update multiselect field

I have a form with a multiselect lookup field "Known_Players" that points to the field "Name" of the same form. When the name of a player is changed I want to update all the Player records in which the name is selected in the Known_Players field. For this I have the following code in the on success / on edit event:

for each player in frmPlayer [Known_Players.contains(input.Name_before_edit)]
    {
        oldList = player.Known_Players;
        newList = List:String();
        for each item in oldList
        {
            if (item == input.Name_before_edit)
            {
                newList.add(input.Name);
            }
            else
            {
                newList.add(item);
            }
        }
        player.Known_Players = newList;
    }

In order to offer the updated name as a choice in the field Known_Players I added the following line in the on user input event of the field Name:

Known_Players:ui.add(input.Name);

This works fine.

But: Udating the list (player.Known_Players = newList;) fails.

Any suggestions?