How to Convert old JPG pics to HIEF in High Sierra?

Hi Guys,

How do you convert all pics in Photos Library on MacOS High Sierra to the new HIEF format to save up some space on the hard drive? I have found external tools but my concern is that if I manually do that manually on the Photos Library, the results will be that I lose the control over those pics within Photos app... In the Photos app in fact I could not even find an option to export to the new format, only JPG, PNG and TIFF.

Is there any thoughts btw about the new format image quality wise?