using get_post_thumbnail with custom size,some work old post does not

added
add_image_size('mmycustomize', 100,115,true);
images that are added today, when queried with get_post_thumbnail(array(100,115))
will work but old posts that had already had images uploaded does not, old attachments are shown as 100X100 because these images are originally square in size,
is there a way to make these old attachments upload to get the new feature as well??