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Yes, always keep or redirect URLs. So often during a website redesign, web designers become lazy and create brand new child page URLs. When the site goes live, those URLs are 404 codes stranded on Google for a few weeks. Terrible practice. As far as on page content, I am a fan of 300+ words as well. However, in a more competitive keyword, I would recommend 1,000+. The jury is still out as to how many pages to update per day. I would estimate 10 per week would be great...but build on the content...not necessarily replace.