Merging Blogs – Permalink issues

I’ve read some great posts about how to merge posts/pages/etc from one WordPress blog into another. One topic I don’t see covered well: Permalink issues.

Blog A is large, about 500 posts, 14,000 comments. I’m going to merge its posts/pages into a newly designed Blog B, which has about 50 posts and pages that I’d like to preserve.

Both blogs use the same permalink structure:
/%post_id%/%postname%

2 questions:

1. When I merge A into B, there will be conflicts in the post_id. Some posts will have the same post_id. Of course, the rest of a post’s name will be different. Should I expect problems because some post_id’s will be the same? Will the Import tool re-number them, or keep the duplicate id’s?

2. Now let’s say Blog B has a different permalink structure:
/%postname%/ (no id)

When I merge in posts from Blog A (which have a post_id), will this be preserved for the A posts, or will the permalink setting of B (the “receiving” blog) cause the post_id from Blog A to be deleted? Does WordPress ever permit imported posts to have different permalink structures than the ones already in that blog?

Thanks for any insight/advice. (And if you’ve got any other cautions about such a merger, I’d welcome your comments!)