My client believes that he just has to copy the data over into the new database using some kind of migration tool and change the app's database configuration file.
While I am sure there are some parts of the app that would still continue to work, I would that bet that there would be a significant number of SQL errors thrown by the new database.
I've tried to explain this but he insists that "SQL databases all the work same way". Unfortunately, we don't have time to prove this by setting up the new database and migrating the data over.
Furthermore, the client intends to do it himself, so I don't see why we should spend any developer time on it. All he is asking for is advice and support when he attempts it himself. How do I tell him tactfully that he will most likely fail?