If you want to change data source from MySQL to PostgreSQL, try data source replace.
If you want to view the custom SQL staments, try open .twb file with a text editer.
Oh I found an ugly solution. I've setup a MySQL server on my local machine and when the prompt comes up I click on Edit connection and type in localhost with id:pass and Tableau lets me view the custom SQL Queries I've written.
Please let me know if anybody knows more elegant solution to replace a single connection element of a data source.