The official demo and launch announcements confirm that demo saves carry into the full game. This is a demo-to-full-game promise, not proof of universal Steam, GOG, PlayStation, Xbox and Nintendo cross-save. Keep the original save intact and use the same platform/account when checking for your existing campaign.
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Safe transfer checks
Update the full game, retain a backup of your demo save and check the available campaign slots using the account that played the demo. Do not delete the demo data as a troubleshooting step. If the campaign is missing, record the platform, game version and save location for support instead of blindly overwriting another slot. No universal manual-copy folder is established by the launch announcement.
Steam achievements
Update 1.006 specifically fixed retroactive Steam achievement unlocks for milestones completed in demo saves. This supports checking the updated Steam build before assuming progress is lost. It is not evidence of automatic achievement transfer between different platform accounts.