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Lowercase, but not just the letters

Given a string of printable ASCII characters, convert them all to lowercase, except not just the letters.

ASCII characters that are letters have a bit in their binary representation that is 0 for upper case, and 1 for lower case. Setting this bit to 1 means that regardless of the previous case of the letter, it will now be lower case. Doing the same for non-letter characters results in some characters changing to a completely unrelated character, which for this challenge we will call the "lower case version".

## Input
- A string of characters, each of which is a printable ASCII character (character codes 32 to 126 inclusive)
- There will never be an underscore `_` (character code 95) as its "lower case version" is character code 127, which is outside the printable range and used as a control code
- There may be spaces (as this is character code 32)

## Output
- A string of the same number of characters as the input
- Each character is either the same as the input, if it was a "lower case version" already, or else the "lower case version" of the input character

## Examples
Character `A` is character code 65, or `1000001` in binary. The bit in position 5 from the right, representing $2^5$, is `0`. Setting this bit to `1` gives `1100001`, or 97, which is the character code for `a`. So the "lower case version" of `A` is `a`, as expected.

Character `^` is character code 94, or `1011110` in binary. The bit in position 5 from the right, representing $2^5$, is `0`. Setting this bit to `1` gives `1111110`, or 126, which is the character code for "~". So the "lower case version" of `^` is `~`.


## Test cases
Test cases are in the format `"input" : "output"`

Note that `"` and `\` have both been escaped with a preceding `\`, because they are enclosed in double quotes, but each still represents a single character)

```text
" " : " "
"!" : "!"
"\"" : "\""
"#" : "#"
"$" : "$"
"%" : "%"
"&" : "&"
"'" : "'"
"(" : "("
")" : ")"
"*" : "*"
"+" : "+"
"," : ","
"-" : "-"
"." : "."
"/" : "/"
"0" : "0"
"1" : "1"
"2" : "2"
"3" : "3"
"4" : "4"
"5" : "5"
"6" : "6"
"7" : "7"
"8" : "8"
"9" : "9"
":" : ":"
";" : ";"
"<" : "<"
"=" : "="
">" : ">"
"?" : "?"
"@" : "`"
"A" : "a"
"B" : "b"
"C" : "c"
"D" : "d"
"E" : "e"
"F" : "f"
"G" : "g"
"H" : "h"
"I" : "i"
"J" : "j"
"K" : "k"
"L" : "l"
"M" : "m"
"N" : "n"
"O" : "o"
"P" : "p"
"Q" : "q"
"R" : "r"
"S" : "s"
"T" : "t"
"U" : "u"
"V" : "v"
"W" : "w"
"X" : "x"
"Y" : "y"
"Z" : "z"
"[" : "{"
"\\" : "|"
"]" : "}"
"^" : "~"
"`" : "`"
"a" : "a"
"b" : "b"
"c" : "c"
"d" : "d"
"e" : "e"
"f" : "f"
"g" : "g"
"h" : "h"
"i" : "i"
"j" : "j"
"k" : "k"
"l" : "l"
"m" : "m"
"n" : "n"
"o" : "o"
"p" : "p"
"q" : "q"
"r" : "r"
"s" : "s"
"t" : "t"
"u" : "u"
"v" : "v"
"w" : "w"
"x" : "x"
"y" : "y"
"z" : "z"
"{" : "{"
"|" : "|"
"}" : "}"
"~" : "~"
"([({Enclosed})])" : "({({enclosed})})"
"A@B.c" : "a`b.c"
```

## Rules
- There is no requirement to use bitwise operations to achieve the correct output
- Provided your output is correct for each input, your code is valid

> Explanations in answers are optional, but I'm more likely to upvote answers that have one.