remove_parent_keep_children.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import lxml.etree as et

xml="""
<text:doc xmlns:text="www.doc.com">
   <text:span text:style-name="Kursiv_5f_KeiinRecht">
      <text:span text:style-name="T25">madhuparká</text:span>
   </text:span>
</text:doc>
"""

ns = {"text": "www.doc.com"}

tree = et.fromstring(xml)

for remove_elem in tree.xpath("//text:span[@text:style-name='Kursiv_5f_KeiinRecht']", namespaces=ns):
    children = remove_elem.getchildren()
    parent = remove_elem.getparent()
    parent.remove(remove_elem)

    for index, child in enumerate(children):
        parent.insert(index, child)

print et.tostring(tree, pretty_print=True)