pipeline - Unix tr command to convert lower case to upper AND upper to lower case -

so searching around , using command tr can convert lower case upper case , vice versa. there way both @ once?

so:

$ tr '[:upper:]' '[:lower:]' or $ tr a-z a-z

will turn: "hello world abc" "hello world abc"

but want do: "hello world abc"

please help! =)

this looking for:

 tr '[:upper:][:lower:]' '[:lower:][:upper:]'