Tickets are date based so anything you buy is going to be tied to those dates.
There is really no reason to buy now because when you go to use them, you’d have to get guest services to change them to the correct start date and if the cost is more, you’ll owe that.
I have not had luck buying tickets through ticket brokers and getting Disney to adjust via phone or through
MDE. I have had to wait until I was there in person.