I have to change the urls of my site, from something like:
This will cause problems with SEO?
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Google allow you to declare the purpose of query string parameters within Webmaster Tools. According to Google:
In general, URL parameters fall into one of two categories:
Parameters that don't change page content: for example, sessionid, affiliateid. Parameters like these are often used to track visits and referrers. They have no affect on the actual content of the page.
Parameters that change or determine the content of a page: for example, brand, gender, country, sortorder.
If Google have already crawled pages with query strings then they may already appear on the URL Paramaters page in Webmaster Tools. Otherwise you can just click to Add parameter for any new ones.
If the parameter doesn't affect the content displayed to the user, select No in the Does this parameter change list, and then click Save. If the parameter does affect the display of content, click Yes: Changes, reorders, or narrows page content, and then select how you want Google to crawl URLs with this parameter.
For more information read the Webmaster Tools help page on URL Parameters.
It depends. If entirely new content is pulled up when that querystring is present, no, you will be fine. If the same content is pulled up with only a minor variation due to that querystring then, yes, you will have duplicate content issues.
Fortunately that is easily resolved by using canonical URLs. Just make the non-querystring URL the canonical URL and you'll be just fine.