PoC

replace-tags.php

function has_class_single_title_container($content) {
	return strpos($content, 'class="single-title-container"') !== false;
}
function replace_text($content){
	if (is_single() && has_class_single_title_container($content)) {
		$content = preg_replace_callback('/<h2(.*?)class="entry-title"(.*?)>(.*?)<\/h2>/', function($matches) {
			return '<h1' . $matches[1] . 'class="entry-title"' . $matches[2] . '>' . $matches[3] . '</h1>';
		}, $content);
	}
	return $content;
}
add_filter('the_content', 'replace_text');