current-markup.html

<!-- CURRENT block markup -->
<div class="wp-block-group cards">
    <div class="wp-block-cover">
        
    </div><!-- .wp-block-cover -->
</div><!-- .wp-block-group -->

<!-- DESIRED block markup -->
<ul class="wp-block-group cards">
    <li class="wp-block-cover card"></li>
</ul>

plugin.php

<?php

function cards_filter_group_content( $block_content ) {
	$processor = new WP_HTML_Tag_Processor( $block_content );

	// Check for the presence of the 'cards' class.
	if ( ! $processor->next_tag( array( 'class_name' => 'cards' ) ) ) {
		return $block_content;
	}

	// Loop through each child block with the class name 'wp-block-cover'.
	while ( $processor->next_tag( array( 'class_name' => 'wp-block-cover' ) ) ) {
		$processor->add_class( 'card' );
	}

	$block_content = $processor->get_updated_html();

	// Enqueue the custom script and style.
	wp_enqueue_style( 'cards-style' );

	// Return the maybe modified block content.
	return $block_content;
}
add_filter( 'render_block_core/group', 'cards_filter_group_content', 10 );