how do I convert "Live" HEIC photos to JPEG?

One way is to export the unmodified original to a folder on the desktop. That will produce two files for each live photo: a .mov file and a .jpeg file.

Gather up the jpeg files and import back into the library.

Missed the "HEIC" in the first post entirely. 🤭

You need a Mac with macOS 10.13 High Sierra or later to be able to convert HEIC to JPEG. (Using HEIF or HEVC media on Apple devices - Apple Support)

Is the system version below your post still valid? Then you need to upgrade the Mac to High Sierra: Upgrade auf macOS High Sierra - Apple Support

On macOS 10.12 you can use the online HEIC Converter (http://heictojpg.com/) to convert the HEIC files to JPEGs, if you do not want to upgrade.