Tickets now have date-based pricing.
So if you buy tickets for October dates, and change the date before your visit, you would only have to pay the difference if the dates you want are priced higher. No refund if you move to cheaper dates.
if you don’t switch the dates before your visit, and the tickets expire, then the tickets retain their value, and you can use that value toward the purchase of future tickets.
if Disney is closed for the dates that you bought tickets for, there will be a different policy. Until that time, no one can tell you right now what the policy would be by then. People who have tickets during the current closure can use them anytime until mid December this year.