Hello Yseael,
Welcome to Asana and the Community!
Unfortunately, it is not possible to prevent people from changing the due date or title, even with advanced permission control.
But you can always see what happens in the activity log of a task
Regarding your migration from smartsheet, there are services like tray.io and Zapier who can migrate the items for you and save you hours of work, let me know if I shall make you an introduction.
Moreover, I recommend you agree with your team on an Asana convention to avoid mistakes in changing things which shall not be changed, find more about this here: Asana Implementation Process: Best practices and lessons learned from different companies - #16 by Dan_Olson
Best