It would mean re-writting every page, as the myODBC database drivers don't fully support ADO so all ADO inserts and updates would have to be changed and written as SQL Update statements etc. which will also mean cerating new security filters and adding extra protection to stop things like SQL injections.
Also mySQL is not a relational database so you would have to cerate the foum databases without the relations.
Many of the SQL statements may also have to be changed as things like Top 1 and sub quiries are not supported.
In other words the forum would need to be totally re-written to use mySQL.
I myself am going to stay away from mySQL until it has more features and they improve the myODBC driver so that ASP/ADO can talk to a mySQL database faster and with more functionality.