yourlist = [ 'Item_has_was_updated_May_2010',
'Item_updated_Apr_2011',
'Item_got_updated_Sept_2011']
x = 'foo'
y = 'bar'
yourlist = [s.rsplit('_', 2)[0] + '_' + x + '_' + y for s in yourlist]
print yourlist
eW91cmxpc3QgPSBbICdJdGVtX2hhc193YXNfdXBkYXRlZF9NYXlfMjAxMCcsCidJdGVtX3VwZGF0ZWRfQXByXzIwMTEnLAonSXRlbV9nb3RfdXBkYXRlZF9TZXB0XzIwMTEnXQoKeCA9ICdmb28nCnkgPSAnYmFyJwoKeW91cmxpc3QgPSBbcy5yc3BsaXQoJ18nLCAyKVswXSArICdfJyArIHggKyAnXycgKyB5IGZvciBzIGluIHlvdXJsaXN0XQoKcHJpbnQgeW91cmxpc3Q=