Converting photos from HEIC to PNG or JPG is surprisingly simple with imagemagick. If you don't have imagemagick installed, you can install it with homebrew on mac like so:
brew install imagemagick
With imagemagick installed, you can now use
mogrify to convert your HEIC files to the desired format. For example, convert a specific HEIC file to jpg like so:
mogrify -format jpg YOURFILE.HEIC
That's it! This simple command will create a jpg file for your image. Note that this will not delete the HEIC file.
Batch Convert HEIC Files
Converting a batch of HEIC files is just as easy. From your terminal, make sure you are in the directory that has the files you want to convert, then again use the mogrify command from above, but this time use an asterisk instead of a single name:
mogrify -format png *.HEIC
This will create a png file for every HEIC file in the current directory. Change this to jpg if you want the files to be jpg instead.