I uppercase the source code, you reverse the input! It's one of these again :D
Your task, if you wish to accept it, is to write a program/function (without any uppercase letters) that outputs/returns its string input/argument. The tricky part is that if I convert your source code to uppercase, the output must be reversed.
For simplicity, you can assume that the input is always a single line string containing only ASCII letters (a-z), digits (0-9), and spaces.
You do not need to handle empty input.
Example
Let's say your source code is abc and its input is hello. If I write ABC instead and run it, the output must be olleh.
05AB1E, 1 byte
r

Try it online lowercase or uppercase!
Finally a question which I (a dumb brain) can answer! Thanks for this easy, yet fun challenge! (I do feel great, even though it requires little effort to make an answer in 05AB1E.)
Wait, how?
r # reverses the stack. (Which literally does not do anything since only the
  # implicit input is in the stack)
R # actually reverses the top string of the stack (Which is the implicit input).
  # at the end, the input is automatically printed.