Just give Disney a call and they can help you modify the tickets to the new start date and you’d pay the difference,
If not bought via Disney, then you will probably have to call them or wait until on-site, I was told that Disney could not help with third party tickets over the phone but it doesn’t hurt to start there if these are the types of tickets you have,
I have also read there is a way to do it through
MDE...I have never had that ability