User photos not showing after replacing photo
Can I ask why you can't change the filename to the old one? For the new file that is. That will easily solve the problem.

The reason it won't show up as per the new file name is that when you upload a profile photo for a user, craft creates a image model for it and when copying over a file directly this image model will not be created hence you either need to use the backend for uploading profile photos or change the filename to the old one. Also you would need to do this in multiple folders as inside userphotos/{username}/ you will see that there are multiple folders, these are folders for different cropped sizes of the profile photo and will be displayed in different locations, Hence you will need to replace the file in all of these locations for the user photo to appear correctly in all places.