After a form is manually uploaded to the server and a change is made to the form, which files need to be uploaded to effect the change, without uploading the entire form folder?

All depends on what you did. At the very least the myformname.html. You may need more if you added other elements that need validation like the validation_data.js.

You would need to upload the entire form files. This keeps you safe from missing that one file and having to upload all the files anyway. :) Also remember to delete and reenter your code on your website when you update your form. :)