When JPG to JPEG turns JPG into JPEG
Some upload forms reject a file whose name ends in .jpg and only accept .jpeg, even though both are JPEG.
What JPG to JPEG changes in the picture
The photo is redrawn in the browser and saved with the other extension. It is a new JPEG save, not a rename of the same bytes. The file is not uploaded.
Each JPEG save softens fine detail a little. If the form only checks the letters in the name, this is enough. Keep the camera original.
When JPG to JPEG is the right page
JPG to JPEG sits on this site because JPG and JPEG are not interchangeable, even when they look similar in a file list. Some upload forms reject a file whose name ends in .jpg and only accept .jpeg, even though both are JPEG. Portals, printers, and chat apps that reject HEIC, WEBP, or AVIF are asking for JPG. A few old upload forms demand the letters JPEG or the letters JPG and reject the other.
The photo is redrawn in the browser and saved with the other extension. It is a new JPEG save, not a rename of the same bytes. The file is not uploaded.
What you give up by leaving JPG
JPG is the photo file almost every form, email app, and Windows program accepts. JPG is lossy. Saving it again softens fine detail. It cannot store transparency.
Keep the camera original, whether that was HEIC, PNG, or a RAW file you exported earlier.
What you gain, and do not gain, with JPEG
The download has to be JPEG, because that is the format the next step named. A few old upload forms demand the letters JPEG or the letters JPG and reject the other. Changing the extension family satisfies a form that only checks the name.
Do not bounce the photo back and forth. One pass is enough.
A test that matches the real file
Start from the question the other software asked. If it asked for JPEG, JPG to JPEG is the matching page. If it asked for something else, switch tools instead of hoping a similar extension will pass. Some upload forms reject a file whose name ends in .jpg and only accept .jpeg, even though both are JPEG. The photo is redrawn in the browser and saved with the other extension. It is a new JPEG save, not a rename of the same bytes. The file is not uploaded. The limit is the part people skip, and it is the part that decides whether the download is usable. Each JPEG save softens fine detail a little. If the form only checks the letters in the name, this is enough. Keep the camera original. The file stays in your browser and is not uploaded. The limit is 100 MB. Keep the original until the download has been opened in the program that will use it. Check three things in the download: it opens, the JPG material you cared about is still there, and the file size or the layout is what the form described. A new extension alone is not those three things. If a portal, a teacher, or a client rejects the file, send them the original JPG as well when you are allowed to. JPG to JPEG is a convenience copy. It is not a new master. Photos fail uploads for three separate reasons: the format name, the pixel size, and the kilobyte size. JPG to JPEG only does the JPG to JPEG step. If the form also prints a pixel size or a 20 KB or 50 KB cap, do that step with the matching tool, starting from the original each time. A second lossy save will not recover detail.
Here is the decision for JPG to JPEG. You start with JPG and you stop when a real program accepts JPEG. It is small and universal for photographs. This is still a new JPEG save, not a rename of the same bytes, so fine detail softens slightly. If those two sentences describe a tradeoff you do not want, do not convert. Renaming the extension by hand will not do this job. Running the tool twice on its own output will not do it either.
Each JPEG save softens fine detail a little. If the form only checks the letters in the name, this is enough. Keep the camera original.
A related step, if you still need one, is JPEG to JPG.
Other jobs start from the homepage, where each tool is named for the conversion it actually does.