Changing Permalinks – SEO impact & Facebook Likes

Hi,
i’m running blog with over 3000 posts. Now i need to change form of permalinks and need to minimize casualties of this change.
I’v already checked that wordpress automatically redirect to new post url when accessing the old one. How this redirect works? Will google see this change?
Second think i need to solve are facebook likes, which are bound to page url. I had an idea to give facebook old formatted urls for posts written up to this date. The question how do i change output of the_permalink() for this posts?

Thanks for help in advance