Characters and stash contents will persist through most updates. There may be exceptions.
In our pre-alpha demo, characters were locked to the game version in which they were created.
The contents of your stash should carry forward to later versions of the pre-alpha demo.
Note: Version 0.4.9.7 implemented different stashes for different types of characters (e.g. Hardcore), and due to this it was unfortunately not possible to migrate stash contents from previous demos.