I have a website which has links like this: www.domain.fi/topic
So all news items get that kind of url, top of that I have some extra url's with the same logic like www.domain.fi/guestbook etc...

I'm currently doing a redesign of the website and thought to switch over to codeigniter at the same time. Now ofcourse I could do the same old url remapping with new site but I want to modify it a bit. Maybe just use the codeigniter default urls so they would be like www.domain.fi/news/item/topic or something like that.

What's my best option SEO wise to do the redesign? It's not the most important site so basicly I could just ignore the whole SEO but still it would be nice to consider that part at least a bit too.

http://codeigniter.com/user_guide/general/routing.html

Sample
Before The route is added.
http://www.stanleywebdesign.co.uk/workings/index/blogging

Route is added


After route is added
http://www.stanleywebdesign.co.uk/blogging

That's not what I needed. I know I can do that with CI but I wanted to know SEO-wise how should I make the transfer to new url's? OR should I?

If the domain is the same do your best to keep existing url's. Google will have indexed your site as it is, if you change all of the links it will have to change the links and this will harm you SEO as you lose backlinks etc.

Yeah, that's what I thought too. I just don't like the current url scheme at all but maybe I just have to live with it. It may be too big job to switch to new url scheme and then support the old ones so that they'll work too.

Codeigniter allows you to execute functions before the controller. You could have an array containing your page titles. Then if the requested URL is in the array the user could be redirected to the relevant page. This way all links will continue to work.

If the URL structure is a hassle or could be better then consider changing it. However to keep the SEO ranking it would be better to support the current url's. Really its up to you. If you feel that the long term gain in changing the URL's is better than the short term SEO drop then go for it.

I only have 2 pagerank (3 for the front page) so I guess I'm not falling from far. I could just make sure that the old links get a decent error page with a link to the front page. I guess that would do it. I'm still considering how to do this. I may keep the old scheme too.