Will Fixing the Metadata Change My Original Photos?
What actually gets written, and whether the picture itself is touched.
Writing a date or a location changes the metadata block at the start of the file. The compressed image data after it is untouched, byte for byte. The picture is not re-encoded and loses no quality.
One thing does change pixels: converting HEIC to JPG, because they are different formats and the image has to be decoded and re-encoded. At a high quality setting the difference is invisible, but it is a real conversion.
Takeout JSON Metadata Fixer can keep every original in a separate subfolder, so you always have the untouched version. It also offers a dry run that shows what would happen without writing anything at all.