Permalinks Migration

My photo blog is quite new with about a dozen (picture) posts.

I’ve been reading the forums and docs about SEO and it would appear to be good sense to change the permalinks to /%category%/%postname%/ from the existing method. I know how to change it, but would appreciate a bit if guidance regarding my earlier posts…

1. Will I need to download the plugin so that the initial posts will still be findable?
2. Or should I just manually change something to achieve the same ends?
3. Or does this version of WP look after it without my input?

Thank you. I’d also like to mention that recently I posted a query with a string of code. I realise now that this is bad form, my apologies to folks.

Seoirse