Hi Graham and Doug,
I migrated from 1.9.17+ to a clean 2.3 last August. Most everything went fairly well. My biggest problem was I had about 370 wiki's made in dfwiki that I had to copy and paste one page at a time because there was no upgrade for it. I actually converted them to books because of the automatic table of contents and links.
The other thing that occured that was a problem, and I did not know it until too late, was I wound up with a bunch of duplicate question banks. Every test that got moved from 1.9 to 2.3 created its own question bank. Since many of the tests were about the same subject, (pre-tests, in-progress test, and post-tests) I wound up with numerous duplcate questions. I didn't realize this had happened for about two weeks. If I had realized how much trouble it would be to get rid of the duplicate questions, I would have did the migration a second time without the tests. As it was, I spent quite a few hours consolidating and eliminating duplicate questions so I would not have multiple copies/versions of questions to maintain.
Hope your migration goes well,
AL