That's not how I read EC261/2004.
If you are on a through-ticket, then Article 5 (1) (a) dictates that you are entitled to Article 8 assistance, regardless of how much notice you have been given.
Article 8 gives you a choice of a refund or re-routing. Article 8 (3) is quite clear about the airline being responsible for transport between airports when you are re-routed to a different one within the same region.