I have a blog with around 40 posts, and i want to change the URL structure these posts. this is my old URL something like: http://domain.com/blog/2013/02/04/string-here.htmli want to set the URL like: http://domain.com/blog/category/string-here/Now the concern is recently i have get the PR to my site after a long struggle ;) i am using wordpress for my blog, and will do url rewriting easily,i just want to know should i have to use 301 permanent redirection so that i will not get the 404 page not found error for my old pages??  
URL re-write for indexed pages
seo;url rewriting
Yes, you should use a 301 Redirect if the pages are going to move to the new URL structure permanently. The new structure is an improvement in terms of readability also.