After importing , I found all posts are re-numbered.
In old site , for example , one post can be directly linked by
/showthreaded.php?Number=77087
In new site , it will be redirected to
/ubbthreads.php/ubb/showthreaded/Number/77087/site_id/1
But the real number is 80713
If I take away the site_id parameter :
/ubbthreads.php/ubb/showthreaded/Number/77087
It will point to different post.
I wonder how do you handle this mapping ? (77087 to 80173)
I cannot find such mapping in table ubbt_POSTS.

BTW , how long will you handle this mapping ?
Will you announce that since 2010 , all old number will be invalid ... ??
And , is it possible to prevent this re-numbering?

It's not possible to prevent the renumbering without some alterations of the import script. The mapping will never go away since it's stored in your local database in the ubbt_IMPORT_MAP table.

The importer was written to allow for multiple imports. The site_id is basically an import id. Since you could actually import several forums into your version 7 installation, and each would have it's own site_id so it knows which post to lookup.