Check if the value of a field has changed on save_post

Usually there is no way to check if the meta data is updated or not, but you can use updated_postmeta and added_post_meta to get kinda similar functionality. Please have a look at the below code-

add_action( 'updated_postmeta', 'the_dramatist_updated_postmeta', 10, 4 );
add_action( 'added_post_meta', 'the_dramatist_updated_postmeta', 10, 4 );
function the_dramatist_updated_postmeta(
    $meta_id,
    $object_id,
    $meta_key,
    $meta_value
) {
    // check meta key with $meta_key parameter
    if ( 'your_meta_key' !== $meta_key ) {
        return;
    }
    // better check with the exiting data with the $meta_value to check if the data is updated or not.
    // then do what you want to do with the data
}

Hope this above helps.