# [Perl 5] `-p`, <s>20</s> 19 bytes

*-1 due to @NahuelFouilleul*

<!-- language-all: lang-perl -->

    $_=lc;y;aeiou;AEIOU

[Try it online!][TIO-k84u3q9a]

[Perl 5]: https://www.perl.org/
[TIO-k84u3q9a]: https://tio.run/##K0gtyjH9/18l3jYn2brSOjE1M7/U2tHV0z/0/3@P1JycfIXw/KKclH/5BSWZ@XnF/3ULAA "Perl 5 – Try It Online"

Convert everything to lower case, then change vowels to upper case.