Hi you could do something like this:
<?php if( get_field('field_one') || get_field('field_two') ) { ?>
<?php the_field('field_one'); ?> <?php the_field('field_two'); // Display the fields not the content ?>
<?php } else { ?>
<?php the_content(); ?>
<?php } ?>
So we check to see if any fields exist and if they do we display them, if not we display the post’s content.
I hope that helps!