keeping old links while adding DB

Hi,

This is the situation:

I have an existing WP site. The permalinks are in the default type – like ?pageid123 for example.

I created a NEW site (multisite) with post slug type permalinks. Now I want to export the new site’s DB and import it into the existing one (the one with the default permalinks). BUT I want some of the old links that exist on that site to remain, SEO concern.

So I imported the new DB XML file, but once I tried to set the new pages/posts a “beautiful” links, via setting->permalinks, it changed ALL the OLD “ugly” links as well.

How do I create new “beautiful” links but keep the old ones untouched?

thanks…