Set default image sizes in WordPress to hard crop

How do I set the medium and large images sizes in WP to hard crop?

In my theme I can set the thumbnail size to hard crop using this:

add_theme_support('post-thumbnails');
set_post_thumbnail_size( 96, 96, true );

But I can see no way to make the medium and large images to hard crop.

Is there possibly a way to remove the medium and large sizes and re-add them using:

add_image_size( 'medium', $width, $height, true );
add_image_size( 'large', $width, $height, true );

You can over write the default like this:

add_image_size( 'medium', 200, 200, true );