Google+ Auto Backup does not upload older photos. How to reset?

I began to use Google+ Auto Backup for the photos, and I silly used the full quality option. So when I run out of space, I decided to switch back to standard quality, so not to use the allocated space. Problem is, the already uploaded photos didn't switch to the standard quality and kept on occupying space. So I deleted them and tried to start over the back up with the standard quality. But it's only uploading new photos and I cannot find the way to make it upload the older photos in standard quality. How to solve it?