John, I have successful migrated Dolphin sites to Cheetah manually. It is a tedious process so it is probably best done leisurely over time. The key to migrating is to first install a new Cheetah site and then start copying the table data. The key to setting up the profiles is to first create all the profile fields in the new Cheetah site that are in the old Dolphin site. This is easier to me than trying to import all the different tables that handle the profile fields. Then you can import the profile data and there will be matching columns in the two tables.
The only real issue I encountered was videos. I had to turn to Deano who instructed me that I hadn't imported the Rayz table(s) for videos.
In the case of third party modules, you convert the modules to Cheetah and install them. This will set up the database structure you need including adding the modules to the sys_ tables that handles stuff for modules so you won't have to worry about trying to import any of that stuff. Then you can export the data from the third party modules and import to the new Cheetah site.
Manually migrating is tedious but it can be done. If someone wanted to hire me to migrate a site, it wouldn't be cheap due to the time involved; I wouldn't be willing to do that work cheaply. A migration script would be nice but I am not sure how easy it would be to write and note, it would not migrate third party modules; they would still have to be done by hand.