How to Turn Already Uploaded Photos into Google's High Quality Format?

If you have uploaded photos to Google Photos and your storage space is full, then you can convert them into Google's high quality format because that does not use the space on your Google Photos. Currently, there is no official method of converting photos from original to high definition free storage.

Make sure your primary account is set to "High Definition Free Storage". To do so, select your photos from your primary account by clicking first photo and "Shift+Click last photo. Click "Share" button and then click "Get Shareable link". You will get a link, copy the link and paste it into a browser window. Now, switch to your second account & Click "Add to Library". Delete all your photos from your primary account. You will see your deleted photos in "Trash". They will store there for 60 days. Now send the photos back again, to do so, select your photos from your secondary account and click "Share" button and then "Get shareable link". Paste the link into the browser and switch to your primary account and click "Add to library" button. This way you can convert photos to high definition free storage. Once you done, check all your photos and videos are converted and delete them from your Trash to reclaim all your space.