Change the modifier keys on the right side without changing the left ones
In setting you can't do that, but with the karabiner program you can. You'll find karabiner here: https://pqrs.org/osx/karabiner/.

Install it and open Karabiner event view. Press the key you would like to change and then click 'Add ... to Karabiner elements' where '...' is the name of the key you want to change. In the program that pops up, select the event that you want to remap the key to from the left drop down. 

Note: This does not work on all keyboards because not all keyboards does send the difference between left and right key. You can tell if your keyboard does in Karabiner event view under the name section after clicking a key. In such case, there's a hardware limitation.