A client is in the late stages (code complete) of a website redesign and unfortunately hasn't implemented 301 redirects to point high traffic pages to the new URL's. As I understand it our only option at this point is to create redirects within the CMS.

Our CMS allows us to do this:

1. www.mysite.com/category/current-page.html will redirect to www.mysite.com/new-category-name/new-page.html
2. The site now uses custom logic on our 404 page to check this list of redirects and if one exists forwards the user to the new-page.html

I understand that using 301 redirects would be the correct way to maintain our page rank but I think that would require a code change which isn't possible.

Question

How will search engines respond to this? Will they wait until the redirect happens and allow us to keep our page rank (authority, trust, etc) or will they see the 404 page and down-rank us? Worst case...will they make our new-page.html start from a rank of "0"?

Thanks for your help.