Help - Error in Permanlinks with Wordpress

Okay, well it isn't an error, more a mistake made on my part.

I am only really just beginning to get into wordpress and blogging, I have been a static html type of guy for years.

Basically on my blogs historically I have always put custom permalinks to:

/%category%/%postname%/

However, in hindsight I actually would much prefer them to be just:

/%postname%/

I know I can go in and change it but what about links that I have pointing to my site which have the old link with the category in it?

Is there a good way of not being penalized in google when I change my permalinks to being just "postname".

I think I have heard that maybe 301 redirects might do the trick, but I have never had to deal with them before.

Could someone advise me on the best thing to do.

Luckily, across all my blogs I only have about 100 posts that will be affected so it will only take me a couple of hours and not weeks to set up any permanent redirects or anything else that I need.

Or is there a better way to go about things.

Thanks in advance.

Joe