主语从句：What I saw in the internet yesterday was a film.
应该是改不了同位语从句,因为同位语是用来解释说明名词内容的,只有一些抽象名词,如：plan,suggestion,idea,fact等才可接同位语从句.这里的名词film显然不具备这个特点.
改为定语从句还差不多：The film that I saw yesterday was in the internet.
英语中几乎所有的名词都可以有定语从句,但是可接同位语从句的名词不多.