DuckCarryChoose what you want to keep
HEIC keeps the original phone image format. Optional JPEG conversion uses the system's image codecs. With Keep originals enabled, both are retained; without it, only a successful photo derivative can replace the destination photo original. A failed conversion keeps the source copy and reports a warning. Do not treat a JPEG as a byte-identical backup of HEIC.
H.264 is real conversion
The video option creates a separate H.264/AAC MP4 through AVFoundation on macOS or Media Foundation on Windows. It does not simply rename MOV or its container label. Video encoding is lossy, so the exact source video always stays alongside the derivative, regardless of the photo originals preference. An MP4 source receives a separate derivative name with (H.264).
Metadata, codecs and Live Photos
HEIC decoding can require installed Windows codecs. The Windows video test machine could encode H.264 but lacked HEVC decoding; missing-codec fallback was checked there. Windows video derivatives do not claim to preserve every QuickTime metadata field; the exact source retains it. Live Photo video halves stay with their photo when that option is selected and are not separately transcoded.
Check representative files
Open a few images and videos in your preferred viewer. Check orientation, dates, sound and the destination folders. Allow extra space: conversions create extra output and video keeps its source. Preflight capacity is an estimate; source sizes cannot precisely predict encoded sizes. Keep an independent backup before considering phone space recovery.