Bash 5.1 provides a straight forward way to do this with the L parameter transformation:

${var@L}

So for example you can say:

$ v="heLLo"
$ echo "${v@L}"
hello

You can also do uppercase with U:

$ v="hello"
$ echo "${v@U}"
HELLO

And uppercase the first letter with u:

$ v="hello"
$ echo "${v@u}"
Hello

Bash 5.1 provides a straight forward way to do this with the L parameter transformation:

${var@L}

So for example you can say:

v="heLLo"
echo "${v@L}"
# hello

You can also do uppercase with U:

v="hello"
echo "${v@U}"
# HELLO

And uppercase the first letter with u:

v="hello"
echo "${v@u}"
# Hello