Is there a preview option when updating a node, like there is when creating a new node?

No, there currently is no preview option when updating.

Preview for updates is a desired feature and is technically possible, but simply has not yet been implemented by a pmdevil. It is also not a priority as substantial updates are discouraged. A substantial update should usually instead be done as a reply, so that people who are trying to follow the thread can notice that something substantial has been added.