header = df[df['old_header_name1'] == 'new_header_name1']

df.columns = header

In [21]: df = pd.DataFrame([(1,2,3), ('foo','bar','baz'), (4,5,6)])

In [22]: df
Out[22]:
     0    1    2
0    1    2    3
1  foo  bar  baz
2    4    5    6

In [23]: df.columns = df.iloc[1]

In [24]: df.reindex(df.index.drop(1))
Out[24]:
1 foo bar baz
0   1   2   3
2   4   5   6