Loop compatibility

I have a theme which uses the old-style loop i.e.

if ($posts) : foreach ($posts as $post) : start_wp();

as opposed to the new-style loop i.e.

if (have_posts()) : while (have_posts()) : the_post();

Will this theme work with WP 2.1? or should I modify the loop code in the theme (is this easy?) or should I go back to old version of WP – I’m not sure when the loop code was altered)?

Any help or advice would be greatly appreciated.