Hi there
Thanks for posting in the Lucidchart Community! You can find information on how to adjust the format of your diagram in this Help Center article. This does not automatically shift your flowchart from horizontal to vertical, but you can adjust the page settings to fit to a vertical page.
If you do need a feature that allows you to automatically make this change, I recommend posting in the Product Feedback section of the community - from here, other users can upvote it and add details of their own.
For more information on how Lucid manages feedback via this community, take a look at this post.
Please let me know if there is anything else I can help you with today!