When specifying an object qualified an attribute, it is the object that becomes उपसर्जन not the attribute. For example, in मालाम् अतिक्रान्तं - अतिमालम् , मालाम् अतिक्रान्तेन - अतिमालेन - माला's विभक्ति remains the same and it is एकविभक्ति and is the one that is उपसर्जन.