What happens when I change an assignment's template?
Any data you've added to it will be retained.
For example, let's say you have a custom template with a custom tab containing a text field. Let's call this Template A.
Your writer put something in the text field in the custom tab.
However, you suddenly decided to switch to a different template, we'll call this template B.
Template B doesn't have a custom tab. What's going to happen to the content of the text field in Template A's custom tab? It'll be retained.
Let's say you switch back to Template A. You'll see that the text field in the custom tab retains its content.
Therefore, you don't have to worry about losing data when switching to different templates for an item even if you have different custom tabs and fields for every template you have.
However, if you cancel an item, all secondary tabs' content (metadata tab and any other custom tabs your template may have) will be cleared. Only the text editor's content will be kept. If you delete an item after canceling it, all data associated with it will be permanently deleted.