Never-used tickets can be held for a credit towards an "upgrade." You should be able to change them yourself on
MDE before they expire, or by calling. Once the tickets pass their expiration date, you may not be able to access through MDE and will have to call. If you have tickets with a start date during the closure, those will automatically extend expiration to Dec. 15, 2020 once your original dates pass.
The credit can be used to "upgrade" -- WDW's definition of upgrade is 1 ticket that costs same or more than the value of the 1 original ticket/credit -- a 1-for-1 basis. So the new ticket can have more days or fewer days, different entitlements (hopper, plus, etc.). Expect to pay the difference if the new ticket costs more; no refund if the new ticket is valued less than the original ticket. Generally speaking, you would not want to use a 9-day PH+ credit for a party or after hours ticket because of the difference in value. You'd want to purchase another multi-day theme park ticket of similar or greater value.