Can I make changes after I launch a mission?
If you're running a manual mission, yes you can!
Even after launching your manual mission, you can add parts, and edit and delete parts that have not yet been opened for scouts. When you create a part you must publish it in Setup in order to then open it to scouts in the Manage page. Once a part has been opened to scouts, it cannot be edited.
And don’t forget: Testing your mission before launch is a great way to catch any errors!