CalcLockAt a glance: Open the HEIC in Preview and export a new JPEG. Renaming a file extension does not convert its contents, and a converted copy should not replace your original.
Confirm that the format is the real obstacle
Read the error or destination requirements before converting. A service may reject an upload because of file size, pixel dimensions, or a network problem rather than the HEIC format itself. If it explicitly requires JPEG, a real conversion is appropriate.
Keep the original and work on a copy. Decide whether the recipient needs a simple still image or something more, such as motion, an editable portrait, or the untouched capture. A conventional JPEG is a viewing image; do not assume it carries every capability associated with the source in Photos.
Open the file in Preview
On your Mac, locate the HEIC source you intend to convert and open it in Preview. Check that it is the correct image before proceeding, especially when several files have similar names. If it is unavailable locally, obtain the complete source first rather than converting a screenshot of its thumbnail.
If Preview cannot open it, confirm the file completed its download and is not mislabeled or damaged. Do not keep changing the extension in hopes that another name will repair the data. A filename is a label, not a transformation of the file's internal format.
Export a separate JPEG
Choose File > Export, select JPEG from the Format menu, choose a quality setting, and save with a new name or in a separate folder. Apple's Preview documentation describes this conversion workflow and the quality control available for JPEG. Keep the first attempt at a quality appropriate for the destination.
Use a neutral, recognizable name such as event-photo-compatible rather than a string of unexplained revisions. If there is a strict byte limit, adjust the exported copy through a measured size-reduction workflow. Do not overwrite the original just to make the folder look tidier.
Reopen and compare the converted image
Close the export dialog and open the saved JPEG directly. Check its orientation, crop, brightness, and small details. Compare it with the source at a similar size, paying particular attention to gradients, fine texture, and text that might reveal compression damage.
Check the saved file's format and size, not just its visible extension. If the destination allows a preview, select the converted file and review it there before completing an upload. A successful conversion on your Mac is useful evidence, but the destination can still apply separate limits or alter its own preview.
Test batches before converting everything
For many photographs, begin with a few representative files: a bright outdoor scene, a low-light image, a portrait, and one with fine lettering. Inspect the output from each before applying the same settings to a whole folder. Keep batch output separate so it cannot silently replace the sources.
Count the finished files and check for naming collisions or missing items. A folder with the expected number of filenames is only a starting check; sample actual image content too. If one type behaves differently, handle it separately rather than assuming all HEIC files have identical characteristics.
Review privacy and future use separately
A JPEG conversion is a compatibility step, not a metadata-removal guarantee. Inspect the outgoing copy for information you do not want to share and review the image for visible private details. Avoid uploading a sensitive original to an unfamiliar conversion website when a local tool already meets your needs.
Preserve the original for later editing or a different output requirement. JPEG compression can discard information, and converting that JPEG back to HEIC does not reconstruct the original capture. Keep the conversion as an intentional derivative with a clear destination and purpose.
Questions, answered
Can I change .heic to .jpg in Finder instead?
That changes the name, not the encoded image. Export through a tool that actually converts the format, then reopen the resulting JPEG and verify it before using it.
Will JPEG keep my Live Photo motion?
A JPEG is a still image. If movement matters, preserve the original Live Photo and prepare an appropriate video separately rather than expecting a still-image conversion to carry it.