Check for the default <!--more--> and change it but leave custom <!--more Custom Text--> alone (c) Christina Arasmo: https://gist.github.com/carasmo/5e9781c3fd37fa9ef29d67f4c8157657

read-more-standard.php

//*Custom Read More link.
add_filter( 'excerpt_more', 'child_read_more_link' );
add_filter( 'get_the_content_more_link', 'child_read_more_link' );
add_filter( 'the_content_more_link', 'child_read_more_link' );

function child_read_more_link() {
	return ' ... <br><a class="more-link button blog" href="' . get_permalink() . '" rel="nofollow">Custom Read More</a>';
}

read-more-var.php

<?php
// don't use
/** ======================================================================================
 *
 * Read More the_content_more_link
 * Check for the default <!--more--> and change it 
 * but leave custom <!--more Custom Text--> alone
 * 
======================================================================================= */
function yourprefix_more_content_more_link( $more_link ) {
	global $post;
	
	if( strstr( $post-> post_content,'<!--more-->') ) { 
		return sprintf( '<a href="%s" class="more-link">%s</a>', get_permalink(), 'Read More' );
	} else {
		return $more_link;
	}
	
}
add_filter( 'the_content_more_link', 'yourprefix_more_content_more_link' );