Yes I want to change the file name.
Example to change 'zps4ed9b387.jpg' to 'lakeside.jpg'.
I used to just double click on the file name in IPhoto & retype the new name.
iPhoto could not change the filename either. iPhoto displayed the filename in the Info panel as the default title of the photo. When you typed a new name into the title filed, you erased the filename and entered a new title. But the filename of the original image file remained unchanged.
The same will happen in Photos, when you type into the title field. Only, Photos is not using the filename as the title , if a photo does not have a title. As long, as you do not add a title, the title field will be blank. In iPhoto you have seen the filename there.