xdotool will definitely handle a simple case like this, this is exactly the sort of thing AutoKey was designed to do.
In AutoKey, you can define a simple script written in Python using the AutoKey API to do this. This can be assigned to a hotkey and will run whenever that hotkey is pressed. It would look something like this (untested.)
field = clipboard.get_selection()
field = field.lower()
which gets the selected text into a variable, converts it to lower case, puts it back in the clipboard, and then pastes it back into the current selection where it originally came from. (If your application window is a terminal, then you'd have to use Ctrl+Shift+v instead.)
Doing it this way has several advantages: It's arguably easier to do than modifying your keyboard. You can easily change the hotkey in the AutoKey GUI. You can define a window filter so the hotkey only works in the windows of your choice. You can turn this functionality on and off at will. And, since you have the entire power of Python at your disposal, you do almost anything else you can think of.
If you had just wanted to substitute one phrase for another, an AutoKey phrase would have done that without writing a single line of code, but since you needed to manipulate the text, a script was required.
Note: Debian and derivative distributions (Ubuntu ...) currently provide a very old version of AutoKey. This is fixed in Debian testing and will be fixed in Ubuntu 20.04. For now, you can easily install the package with these instructions.
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