I will shortly be moving an e-commerce website from an asp based system onto a php based system.
When I do this I would like to limit the impact on search engine rankings.
As a result I have so far come up with 2 options:
1. Do a permanenet redirect all asp pages to the new matching php version.
i.e: page.asp?id=12 -> newpage.php?id=123
However, this could be a real pain for product pages as their id in the new database could be quite different (as shown above) and would require lookups to the old catalog data to achieve this.
2. Start again. Remove all asp pages completely, resulting in 404s and hope the search engines spider the new site structure nice and quickly!
However, I have noticed in Google that a number of pages exist in their SERPS, even though they have long been deleted. So would adding a new page (with the same content) result in a duplicate content penalty?