Changing Permalink and SEO Affects…Help!

OK, here is my problem. I am using Tiga Theme 1.0.1 with WP 2.3.2 (Yes I know I need to update) When I change to “Pretty Permalinks” the layout of the site is completely whacked. I have a feeling it is in the CSS somewhere, but am no expert there. Soooooo, I have decided to hire somebody to fix the problem as I really like the Tiga theme, and have spent hours tweaking it just so.

My concern is that when I finally am able to use “Pretty Permalinks”, what will this do to my SEO rankings? All of the posts, 2,700 + will have “real post” urls http://www.nevisblog.com/Amesty International Condems Hanging In St Kitts – Nevis instead of the http://www.nevisblog.com/?p=2773 that I now display.

ANY advice on this topic would be just great.

Looking forward to hearing from all of you WP/SEO Gurus.

Happy Holidays!