1. WHEN did you buy the tickets?
2. Either go ahead and just use your current tickets and lose the value of the last 2-3 days (actually, not a major amount of money, all things considered.)
-Or-
SAVE these current tickets, unused (yes, their value can be used to purchase tickets for another trip in the future.)
Then, BUY NEW tickets (with the correct number of days) for use on THIS trip.
Your current park reservations will still work with new tickets.
3. First-off, all current WDW tickets are sold with an ultimate end expiration date.
So, if your current tickets will not expire within your new timeframe, you can change their dates and use them. (If the dates that you do finally go are listed at a higher cost than your currently-booked dates, you will have to pay the difference when you CHANGE the dates.)
-Calling Disney will help you change the dates.-
OTOH, if your current tickets WILL EXPIRE regarding your future new timeframe, you can use the value of those expired unused tickets towards the purchase of new tickets.
The only thing you NEED to do is KEEP YOUR TICKET NUMBERS STORED on your computer, because expired tickets will "disappear" in your MDX account.