Could you make the value the url instead?

<select id="filterSelect" onchange="changeFilter()">
    <option value="<%= user_path(@conn, :index, filter:1) %>">All</option>
    <option value="<%= user_path(@conn, :index, filter:2) %>">Organizer</option>
    <option value="<%= user_path(@conn, :index, filter:3) %>">User</option>
  </select>

<script>
     function changeFilter() {
             var filterUrl = document.getElementById("filterSelect").value;
             window.location.replace(filterUrl);
     }
</script>

Another possibility is to create a javascript object in your eex template. It would have the numbers as the keys and the urls as values. You could then use that in your javascript code

<select id="filterSelect" onchange="changeFilter()">
    <option value="1">All</option>
    <option value="2">Organizer</option>
    <option value="3">User</option>
  </select>
<script>
  var urls = {
   1: "<%= user_path(@conn, :index, filter: 1) %>",
   2: "<%= user_path(@conn, :index, filter: 2) %>",
   3: "<%= user_path(@conn, :index, filter: 3) %>"
  }
</script>

<script>
     function changeFilter() {
             var filterValue = document.getElementById("filterSelect").value;
             window.location.replace(urls[filterValue]);
     }
</script>