Rename first row in a CSV file

[James_S]

Input (my_data.csv):

39, AA, BB, CC, DD, EE
40, 111,222,333,444,555
41,666,777,888,999,000
42,121,232,343,454,565
43,244,355,366,377,388

Expected output (renamed_my_data.csv)

39, aaa, bbb, ccc, ddd, eee
40, 111,222,333,444,555
41,666,777,888,999,000
42,121,232,343,454,565
43,244,355,366,377,388