Change the weight of layout with an animation

In my main layout file, I have a RelativeLayout, with a weight of 1 (basically to display a map) above a LinearLayout with a weight of 2, declared this way :

<LinearLayout
    android:id="@+id/GlobalLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <RelativeLayout
        android:id="@+id/UpLayout"
        android:layout_width="match_parent"
        android:layout_height="0px"
        android:layout_weight="1" >
    </RelativeLayout>

    <LinearLayout
        android:id="@+id/DownLayout"
        android:layout_width="match_parent"
        android:layout_height="0px"
        android:layout_weight="2"
        android:orientation="vertical" >
    </LinearLayout>

</LinearLayout>

DownLayout contains a list of items, when I click on an item, I would like to change the weight of DownLayout for 4, so the upper layout (the map) takes only 1/5 of the screen instead of 1/3.

I have managed to do it by changing the LayoutParams :

    LinearLayout linearLayout = (LinearLayout) mActivity.findViewById(R.id.DownLayout);
    LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(
            LayoutParams.WRAP_CONTENT, LayoutParams.MATCH_PARENT);
    params.weight = 4.0f;
    linearLayout.setLayoutParams(params);

It works but I'm not satisfied, the change is too immediate, there is no transition while I would like it to be smooth. Is there a way to use animation for that ?

I found some examples with ObjectAnimator to change the weightSum, but it does not do want I want (if I change only this property, I have some free space below my down layout) :

        float ws = mLinearLayout.getWeightSum();
        ObjectAnimator anim = ObjectAnimator.ofFloat(mLinearLayout, "weightSum", ws, 5.0f);
        anim.setDuration(3000);
        anim.addUpdateListener(this);