lucy24 - 12:48 am on Dec 30, 2012 (gmt 0)

So my question is, recently re-written urls for all inner pages and updated other details now I want to know what to to do with existing pages ? Should I remove and put 404 or shall I redirect [301] to the new urls from old urls?

The first sentence makes it sound as if you haven't changed the pages themselves, only their URLs. If you did remove the old URLs it would be 410 rather than 404, because they used to exist and you've intentionally removed them. 410 vs. 404 makes a big difference to google's crawling, though it has no noticeable effect on bing. At least not within my small-scale personal observation.

If it's same page, new URL, you would normally redirect. You want your customers to be able to find you from any existing bookmarks or links. Unless you've deliberately renamed the page to get rid of unwanted baggage at the old location. This doesn't seem to be the case here. (Which is just as well, because there are long threads talking about whether this kind of renaming works or not.)

It may depend on what exactly "updated other details" means. Do you want to start from scratch, or keep what you've got and try to improve on it?