If I were to write that sort of tool, I would probably try to use the table definitions as returned by the database servers themselves rather than a SQL script, as the database servers will return the data in a more regular fashion (easier to parse), and in some cases (e.g. Sybase) you don't have to parse anything at all, you just query the system tables and get the necessary data.

Second, I would not go and directly update the target database. Instead I'd create a SQL script that has the ALTER TABLE statements so that it can be checked, and that the update operation is run in a controlled manner.

Michael