I think there is an upgrade/downgrade path between MySQL and DB2 because they are both pretty ANSI standards compliant.
Other than SQL Express, is there a similar pairing of free-"ish"/paid databases for SQL Server?
Another way to state the question-- of the free db engines that exist, which is the least painful to migrate T-SQL to?
Update-- Background: I'm looking for a suitable downgrade path. I wrote an app that I wanted to post to codeplex and then I realized that the likely audience might not have admin rights, wouldn't be able to cope with the administrativia of MS-SQL, etc.