If the rail ticket is an dicounted one, changes are not always possible. So the discussion about cutting a stay short might be like we say in Germany "eine akademische Frage" ("a moot point") .

And though I agree that Rothenburg is not the biggest place, you could spent there more than a night. Given the possible jetlag, this might give you indeed some good times for photos in the early morning with less people around there. Many if not the most tour groups do not overnight in Rothenburgm they arrive around 10:00 / 11:00 and leave around 15:00 to 17:00 depending on their schedule. If you really are into pictures, I recommend two days just to be on the safe side for picutres without too many people. And I wish you good weather.