Is it possible to copy diagrams so that changes can be made without having to start over?
You can't copy a complete diagram, but all the objects you have created on the diagram (decision, sources, etc.) are in the database and can be re-used. Create a new diagram and using the search on the left you can add the objects you have already created.
Here is a short demo that demonstrates creating a new diagram that resuses decisions, etc.
This question has received the maximum number of answers.