If you would like to keep the Sims you created with The Sims 4 CAS Demo, you will either have to transfer them manually between folders, or upload them to the Gallery. The latter is simple and doesn’t require instructions.
To manually transfer Sims, locate the game file for The Sims 4 CAS Demo:
• Documents > Electronic Arts > The Sims 4 CAS Demo > Tray
• Copy the contents of the “Tray” folder
• Documents > Electronic Arts > The Sims 4 > Tray
• Add the contents from the CAS demo “Tray” folder to the full game’s “Tray” folder