Once the voucher expires you can still make changes to your upgraded reservation, provided the new reservation has J fares/seats available and the upgraded segment(s) don't change, it can even be re-routed, as long as the first 2 conditions meet.

The scenario above is called upgrade carry over, now if you want to credit the voucher and change upgrade routing, this would require voucher re-deposit and re-use, with $25 fee, just like miles. Obviously if it's expired and you re-deposit, the voucher will be invalid.

p.s. If you ever have to do this, please try to get it done at competent ticketing office, rather than over the phone.