Automator to change file extensions

You cannot change a file format by changing the file extension. You might be able to run a shell script to modify them with sips: https://blog.smittytone.net/2019/10/24/macos-image-manipulation-with-sips/

Choose Workflow.

Then add these actions, as I described previously:

Ask for Finder Items

Copy Finder Items**

Change Type of Images (select JPEG as output)

** NOTE: the Copy Finder ITems action saves a copy of the original HEIC file. If you don't do this, the HEIC is overwritten by the JPEG output file.

When you run the workflow you will be prompted to navigate to & select a file to convert (or multiple files if you selected that option in the Ask for Finder Items step. You can save the workflow.

HEIC images must be converted to JPG. As @Barney-15E already noted you cannot just change the file extension.

You should be able to use Automator to do this. Create a new Workflow with two actions - add the "Ask for Finder Items" action followed by the Photos > Change Type of Images action where you set the output to JPEG.

Be careful to test this with just a few files in a specific folder before applying it broadly.

I do not advise trying to reduce the size of the JPEG files; you will degrade the image quality by doing so.

Note: the above actions allow you to manually select specific files to convert.

If you want to convert in batches (or all) then you might use Find Finder Items instead of Ask for Finder Items ... but you must then specify the Search location, "All of the following are true," and Kind-Is-other-HEIC

But be careful to test this first on a small batch in a specific location. You want to make sure this works correctly before applying it broadly. Caution is important here ... you can do great damage if it's not done correctly.

Also note that in the Copy Finder Items step you can specify a folder where the copies will be saved; this could be used to aggregate all your original HEIC files in one place.