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Hi Safia, If you saw an immediate change in rankings then there probably was an issue or something affected your rankings before you upgrade and installed the SEO plugin. Google is in most cases not able to directly crawl your site and recalculate the index for specific keywords.

In a word: No. By doing that you a) get closer to being over optimized and b) your current URL has a PR of 4 and decent page authority. You'd be giving up both if you change the URL.By all means change the Title tags and meta description, but changing url WILL hurt you more that it will benefit you.