Hi there,
I'm in the process of upgrading my Tiffany e-ring. From my experience the upgrade value is based on amount paid less a fee (generally 500-1000) assuming great condition of your existing ring, regardless of time you've had the ring. Generally the upgrade is supposed to happen within 5 years but from my SA they have made exceptions up to approx 10 years. This amount must be applied to a ring of 2x value of the original ring. Hope this helps and happy upgrade shopping!