Specifically what tickets are you inquiring about?
Disney tickets as a rule do not expire until they are activated (meaning the first time you go through a turnstyle), unless you have purchased the no expiration feature, then they will NEVER expire.
That being said, there are no refunds on Disney tickets if you bought them directly through Disney if you have already recieved them the probably Disney won't refund them, if you have yet to recieve them you should be able to get a refund (at least Orlando Fun Tickets would).
I hope this helps!
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Jeez, I was going to try to jump in here but Eric seemed to cover it all.
One thing about the special event hard ticket nights is that sometimes guests have been able to change dates and/or events with those but that isn't a guarantee.
If you are debating about adding the non-expiration option and are planning to use all days on your visit my suggestion is always no. You can always add it on later. Yes, it's a bit of a pain to do that later v. initially, but if you don't end up needing it then you are saving money.
The only thing I'd like to add is in my own experience I've had tickets exchanged but never refunded.
I found an old Play 3 Days pass that was never used and they told me it's still worth the dollar amount that you originally paid. So I was able to apply the amount to a park pass, maybe that route might work for you.