5.9.2 Upper/Lower case conversion

GNU sed’s substitute command (s) supports upper/lower case conversions using \U,\L codes. These conversions support multibyte characters:

$ printf 'ABC\u03a3\n'
ABCΣ

$ printf 'ABC\u03a3\n' | sed 's/.*/\L&/'
abcσ

See The s Command.