I have been searching for possibilities to define different shapes inside a single shapes.xml and refer to each one on some specific events.
At last I've found a solution to my question. And the answer is using level-list.
<?xml version="1.0" encoding="utf-8"?>
<level-list xmlns:android="http://schemas.android.com/apk/res/android">
<item android:maxLevel="0">
<shape xmlns:android="http://schemas.android.com/apk/res/android"
android:shape="rectangle">
<gradient android:startColor="#aaa"
android:endColor="#eee" android:angle="270" />
<corners android:bottomRightRadius="7dp"
android:bottomLeftRadius="7dp" android:topLeftRadius="7dp"
android:topRightRadius="7dp" />
</shape>
</item>
<item android:maxLevel="1">
<shape xmlns:android="http://schemas.android.com/apk/res/android"
android:shape="oval">
<gradient android:startColor="#eee" android:centerColor="#ddd"
android:endColor="#00fff2" android:angle="270" />
<corners android:bottomRightRadius="7dp"
android:bottomLeftRadius="7dp" android:topLeftRadius="7dp"
android:topRightRadius="7dp" />
</shape>
</item>
</level-list>
Apply this to the background attribute in the style. the Interchanging of differents shapes can be achieved by setting the level to that element.
Eg: findViewById(R.id.mybutton).getBackground().setLevel(1);
In the above code I'm setting the second shape to the button with id mybutton.