Whenever I update the sitemap for my forum I do it manually, It is not like that I just add the new urls at the top , the order of urls for the whole sitemap gets changed so in effect it is a totally new sitemap, I just want to ask that can it have any negative effects ?
I suppose Google does read urls from the top... Still, I am 99% sure they read the complete xml sitemap before deciding which pages to index and crawl.
I don't think it will affect your site. Though google will read your sitemap file, but to crawl which links provided by sitemap is decided by googlebot based on their algorithm, not by your order.
Yeah, I don't think the order makes any difference and if you don't have a changefreq tags, you can just resubmit your updated sitemap to your acount at Google or Yahoo.