[laimas]

Может проще:

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>
<script>
$(function() {
    $('#srochnost').on('change', function() {
        if(!this.value) return;
        $('#rejim option').slice(0, -1)
                          .prop('disabled', this.value==2)
                          .end()
                          .eq(3)
                          .prop('selected', this.value==2)
    })
});
</script>

<select class="form-control" name="srochnost" id="srochnost">
    <option value="" disabled>Выбрать...</option>
    <option value="1">Срочный</option>
    <option value="2">С проживанием</option>
</select>
 
<select class="form-control" name="rejim" id="rejim">
    <option id="rejim-opt-null" value="">Выбрать...</option>
    <option id="rejim-opt-day" value="1">Дневной</option>
    <option id="rejim-opt-night"value="2">Ночной</option>
    <option id="rejim-opt-sutki" value="3">Суточный</option>
</select>