38k 404 errors in wmt

Recently launched Magento web store... Long story short I had to regenerate URLs for products and also adjusted the way our catalog list page URLs were structured (drill down urls, etc). This was all done relatively quickly after site launch, there are no links that I can see pointing to these pages including the sitemap, but they continue to show up in WMT as 404 errors. I am afraid they are hurting rankings/quality score.

Anyone have any advice, or know for sure if this is really a problem? Its been ~1-2 months since these have shown up, I feel like they should have already gone away. Definitely super annoying.

Redirect the old URLs to the new ones and it will clear right up. Until then you have orphaned your backlicks to all the old URLs.