You will need to transfer your demo save data manually.
Please follow the steps below.

1. To prevent Steam Cloud syncing, select the game in the Steam launcher library, click on the cog in the upper right, and select Properties.

2. In the General tab, turn off "Keep game's saves in the Steam Cloud for Another Eden Begins."
If you have already launched Another Eden Begins on Steam, proceed to step 3. If not, proceed to step 4.

3. Delete "C:\\Users\\<Username>\\AppData\\LocalLow\\WRIGHT FLYER STUDIOS\\AnotherEdenBegins\\SaveData" (if it exists).

4. Copy "C:\\Users\\<Username>\\AppData\\LocalLow\\WRIGHT FLYER STUDIOS\\AnotherEdenBeginsDemo\\SaveDataDemo."

5. Paste the above folder into “C:\\Users\\<Username>\\AppData\\LocalLow\\WRIGHT FLYER STUDIOS\\AnotherEdenBegins\\"

6. Rename "SaveDataDemo" to "SaveData."

7. Launch the full game.
Once save data has been successfully transferred, enable Steam Cloud as needed.

You will need to transfer your demo save data manually.
Please follow the steps below.

1. Press the STEAM button, select Settings, and scroll down to the Cloud tab. Turn off "Enable Steam Cloud."

2. Return to the home screen, press the STEAM button, and go to Power. Select "Switch to Desktop."

3. Select the file manager (default: Dolphin) to go directly to the compatdata folder.
If you are not directed to the compatdata folder, click the ≡ icon and select "Show Hidden Files."

4. Go to Home > .local > share > Steam > steamapps > compatdata.
If you have already launched Another Eden Begins on the Steam Deck, proceed to step 5. If not, proceed to step 7.

5. Go to 4163960 > pfx > drive_c > users > steamuser > AppData > LocalLow > WRIGHT FLYER STUDIOS > AnotherEdenBegins.

6. Select "SaveData," press L2 or right click, and select "Move to Trash" to delete the folder.

7. Click the ≡ icon on the upper right of the file manager, and select "New Tab."

8. Go to 4391790 > pfx > drive_c > users > steamuser > AppData > LocalLow > WRIGHT FLYER STUDIOS > AnotherEdenBeginsDemo.

9. Select "SaveDataDemo," press L2 or right click, and select "Copy."

10. Return to the AnotherEdenBegins folder in the file manager window.

11. In an empty space, press L2 or right click to paste the folder, check "Apply to All," and select "Write Into" or "Overwrite."

12. Press L2 or right click on "SaveDataDemo," and select "Rename."

13. Press the STEAM button and the X button to open the on-screen keyboard, and rename the folder "SaveData."

14. Press the B button to close the keyboard.

15. Close the file manager window, and select "Return to Gaming Mode."

16. Launch the full game.
Once save data has been successfully transferred, enable Steam Cloud as needed.