Is there any way to have macOS Photos bulk convert all of my past JPEG images to HEIC format?
Unless somebody writes a plugin to manipulate photos in place, converting all the JPGs outside of Photos, deleting them from the library and then reimporting seems to be the only way. Which of course means that you loose all the albums etc.
As for a plugin, I wouln't be surprised if this is nearly impossible to do as plugins usually manipulate just the content of a photo, they change neither file type nor file name. And with the in-place editing Photos provides (and the versioning/edit history this requires) even a hack accessing photos.db directly will be challenging to do.