I had the exact same question. My MacBook (13-inch, Mid 2009) can only run OS X 10.11 El Capitan and I really want to be able to take advantage of the storage improvements with the HEIC file format. I ended up using this little tool called the iMazing HEIC Converter which is some free software you can download from their website directly (for both Mac and Windows) or from the Mac App Store. You can give it as many files as you want and it asks where you want to put the processed JPEGs or PNGs, what quality you want them saved at, and if you want to retain the EXIF data. It's also free, which is literally the best part.
Reading through this a second time, it really feels like an infomercial... 😅