Somewhere here on SQAForums (and probably multiple times), I've answered something along these lines.
In a nutshell, think CRUD:
C is for Create. Can you 'create' new data in the migrated database though the application?
R is for Read. Can you 'retrieve' data from the migrated database? (i.e., can you log in with a pre-existing user record)
U is for Update. Can you 'update' existing data? Can you update new data?
D is for Delete. Can you 'delete' existing data? Can you delete new data? (Though, you'll have to figure out if deletion is a meaningful exercise...sometimes, data really changes state such that it appears to be deleted to the end-user...but the data continues to exist in the database.)
Beyond that, are there database-related triggers/views/etc. that you have in your app? Probably need to figure out how to exercise them as well.
"The single biggest problem in communication is the illusion that it has taken place."
-George Bernard Shaw, Irish playwright and Nobel Prize winner, 1856-1950