Migrating written queries

[David Brama]

Hello Everyone.
We are considering migration from SQL server to Mysql. We have a few terabytes of databases, about 1500 tables in each. The migration of the schemata and data went pretty smooth, but we have a major problem migrating our website code. It's written mostly in asp and VB6, and consists of hundreds of thousands of pages with tenths of thousands of SQL queries, many of those are MS-SQL specific containing expressions like "SELECT TOP, isnull(), convert(), FOR XML", and so on.
It is not practical to manually rewrite all the code. some of it is ancient and is not properly maintained. So I have came out with a few ideas:
1. Messing with the server or the connector code to reparse all statements in real time
2. Scripting automatic rewrite of all our code to mysql compatible syntax
3. Using third-party software to convert all our code (Ispier comes to mind)

Do anyone have any experience with migrating projects of this magnitude? can someone suggest a solution? Is it even practical?

Thanks
David