Just wanted to give an update from my own experience today:
background:
I had 2 7-day PH Plus tickets assigned to friends I manage on my
MDE account.
These were Unused, Expired tickets from 2017.
I was told by chat support and a previous phone call that it would require an in-person visit to apply these towards new tickets (I think part of the problem is I incorrectly used the word "upgrade" or "reactivate" when that's not what's really happening). I was trying to get two new 7 day PH Plus tickets with a start date of 9/5/2020.
My brother is at the parks right now, and is marked as allowed to Plan for the 2 friends who these expired tickets were on. Despite this, Guest Relations told him that they'd need ME to do this in person; the best they said they could do for my brother was use them towards new tickets with a start date of TODAY.
So I tried one more time myself: I called "the Disney Reservation Center at (407) 939-3476 " per this page
https://disneyworld.disney.go.com/faq/tickets/ticket-dates/because this page spells out:
Additional Information on Tickets Purchased Before October 16, 2018
The amount paid for an unused, expired ticket may be applied towards the purchase of a new ticket at the current price so long as the new ticket purchase price is equal to or greater than the amount paid for the original ticket.
I basically stated that I wanted to do this^, verbatim.
And lo and behold, they were able to help me do this all over the phone after all!
I even got a better rate than I expected:
The expired tickets were worth $532.50 with tax back in 2017 from what I could tell.
$583.62 is the new price with tax for a start day of 9/5/20. So i thought it should be $51.12 each to get new tickets.
They quoted me a higher value for the dead tickets, and I only had to pay $59.64 total! I'm not entirely sure why but I'm very pleased with the outcome.