Can a photo created as .TIFF be converted to .JPEG?
can photo formats be changed once created?
Mac mini 2018 or later
Question marked as
Photos does not allow to modify the original file inside photos. It is always preserving the original.
You can change the format by exporting the TIFF file from Photos (File > export > Export ... photo) and selecting the JPEG format on export. The reimport the JPEG and delete the TIFF version.