How to Find and Convert the Original Size Photos on My Google Photos Account to Free Some of the Storage Space?

Update:  A button appeared called "Recover Storage" for this:
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This is a big question for a lot of us. Scroll down to the answer if you already know the background of this question. 

Background

With the release of Google Photos at Google I/O 2015, you now have "unlimited" storage for all your photos and videos.
But there are limitations you can read about here:

http://photos.google.com

But your photos you uploaded before the announcement are still taking up space in your google account.  Even the small ones you may have uploaded.

Hence the question: How do I find and convert the original size photos on my Google Photos account to free some of the storage space?

I wish there was a button you could press for this, but I've looked for a long time and as of this answer (July 2015) Google does not have a way to automatically convert your existing photos in their system to the free storage "high quality" setting.  

Another way to find and convert the original size photos on my Google Photos account to free some of the storage space?

Reduce the size of your photos & videos
To clear space in your Google Drive, try converting your backed up photos and videos to High quality, which is a reduced size.

Using a computer, go to photos.google.com/settings.
Click RECOVER STORAGE.