Change jpeg to HEIF

You can convert single image files to HEIF using the Image2HEIF converter from the App Store. Only, this converter does not yet copy the EXIF data to the new HEIF image file. You would lose the the camera tags for lens, camera model, exposure time, etc. HEIF is not yet well supported on High Sierra. Even Apple's own Preview application cannot yet save images as HEIF, and Photos cannot do it either, only, if you import HEIF originals.

I would wait for the next major update of High Sierra, until Apple allows Developers to create software that can export HEIF.

GraphicConverter (from the AppStore) offers a photo editing extension for Photos, and allows already to import HEIC and to process it. It promises that the next update of GraphicConverter will allow to export HEIC,with the caution "will be enabled by Apple with an Update of macOS 10.13".

You can not convert anything within the Photos library (you can NEVER make any change to the content of the library) - to do that conversion you must export the photos even after Photos can do it and use them outside of photos or convert them, delete the JPEGs using Photos and reimport the HEIF photos - basically you need to start with HEIF photos and the Apple IOS devices are the only cameras that do that today as far as I know

LN

I just tried again the most recent version of GraphicConverter, to see, if the HEIF conversion will include the EXIF data, but still no joy.