Previews not working after mapping domain / Blog URL

We had a blog up at one domain (say firstdomain.com), but designed a new site from scratch on a subdomain (new.firstdomain.com). When the new site was ready, we:

1. Re-mapped firstdomain.com to the new.firstdomain.com directory.
2. Changed the new site’s WP Blog URL to firstdomain.com.
3. Updated all links in posts, pages, plugins and custom files to firstdomain.com.

From the visitor’s perspective everything looks great and runs smoothly. However, we can’t view post previews without adding the new subdomain manually to the address. It produces errors that say: No posts exist in this database (for drafts) and You do not have permission to view drafts (for published posts).

It obviously generates permalinks with firstdomain.com, but previews (and thumbnails) with new.firstdomain.com. Does anyone have any idea how to fix this? Did I put this together like a drunk mechanic?

Thanks in advance…