Change Permalinks

Hi,

I am finishing the website and now i know about the default permalinks are not good to the SEO, caches,.. and I need to change the default for the “post name”. But If i make this chance at settings, all the pages have error 404, how can fix this fail about the change links?

Thanks?

PD: I read about 301 redirect, but i don’t find the code to redirect default permalink to post name.