Changing Permalinks – how does this affect old posts?

Hi, I have a question I can’t find the answer to on the forums. We are having problems getting facebook to pull thumbnails and text when we link to a post on our blog. I added a WP-OGP plugin to help, but I understand that Facebook doesn’t like the “Default” permalink setting in WP.

So… if we decide to change to one of the other Permalink settings (year/month/day/id) does that affect posts backwards (our past few years of posts)? As in, will any previous facebook or email list links we have made/sent out to eg.”http://…/?p=3494″ no longer be valid and give a 404 error if people try to visit those old links? Or… does selecting a new permalink setting only affect new posts going forward?

thanks in advance for any suggestions or help!!
Stephen