and how are you going to put the Stored Procedures in MySQL ?
the game will be useless without it since 80% of it depends on the stored procedures in MSSQL
getting the game to talk to MySQL is relatively easy (ODBC links) however. converting the gamedata (and by that i don't mean the table's/data but the procedures/functions and datatypes) will be nearly impossible
the server itself doesn't really read into the database.
it just constantly executes and call's stored procedures
example. if ingame you make a new character
it executes character..SP_CHAR_EDIT_CREATE with the info you've enterred "class,charname"
SP_CHAR_EDIT_CREATE then checks if the name exists, if its allowed, then makes an entry in user_character and reports back to the game with an integer -20 - 20
depending on those integers the game either reports an error or a success