While rummaging through The Sims 4’s Origin Pre-Load Files, I noticed this boxart of the game:
This means that after The Sims 4 is released, you’ll be able to upgrade your The Sims 4 Standard/Limited Edition to Digital Deluxe. The cost for SimCity’s Digital Deluxe Upgrade is 19,99$, so I don’t expect this price to be changed for The Sims 4 as well.