Automatically convert images in Photos.app
Solution 1:
You can select all the png files in your photo and then under the File menu select Export. The top option will be to Export x items where x is the number of png files you selected. Select that option and a new pane appears where you can set the type as jpeg. See pane below.
I'm not positive this saves the metadata so I advise you to run this on one image and see if it works.