I've avoided this for a long time. Finally I launched into it to find what it would take. You'll notice the web site looks a bit different. That's mostly because I didn't try to migrate the old theme; I'll eventually go through and tweak things until it morphs back into something closer. But that's not the painful part.
There's really no clean way to "upgrade" from Drupal 6 to 7. Or if there is, it's very well hidden. Finally, I found the various components of the Migrate API and modules. They mostly get you there. I was even able to migrate all my old events into the new structures and they came over cleanly. But as a user of the various old image modules, I was pretty frustrated. Most of them don't come over very cleanly. I spent a long time editing old nodes to relink images. The Gallery2 node with rotating images is gone, maybe never to return. Of course Gallery2 has been orphaned, so that's another problem. But the biggest pain that shouldn't have existed? All book connections were lost.
So I had to go through and rebuild all the books. Every one. I can't imagine what it would have been like if this had been a truly large web site. You would think that a migration procedure that automatically recognizes the old site has books and the new site has books and would be able to pull in not just the content, but the structure. But, no, it doesn't. I actually set up a test site and migrated into that, rebuilt all the menus, book hierarchies, blocks, everything. Then I tried to do a Drupal 7 to Drupal 7 migration. It didn't do any better. Blech.