from xml.etree import ElementTree as et

def replace_element(elem, replacement): # tested
    '''replace the content of an ElementTree Element by that of another one.

        >>> original = '<a b="c">d<e>f</e>g</a>'
        >>> new = '<aa bb="cc">dd<ee>ff</ee>gg</aa>'
        >>> elem = et.fromstring(original)
        >>> replacement = et.fromstring(new)
        >>> elem_id = id(elem)
        >>> replace_element(elem, replacement)
        >>> print(elem_id == id(elem)) # same object as before
        True
        >>> print(et.tostring(elem) == new)# but with new content
        True
        >>> print(new)
        <aa bb="cc">dd<ee>ff</ee>gg</aa>
        >>> print(et.tostring(elem))
        <aa bb="cc">dd<ee>ff</ee>gg</aa>
    '''
    elem.clear()
    elem.text = replacement.text
    elem.tail = replacement.tail
    elem.tag = replacement.tag
    elem.attrib = replacement.attrib
    try:
        elem[:] = replacement[:]  # quick and works with Python 2.x but not 3.x
    except AssertionError:
        #print("Assertion error")  # uncomment for testing
        del elem[:]
        for child in replacement:
            elem.append(child)

import doctest
doctest.testmod()