You can buy the tickets now but there are some considerations...
if Disney comes out with some sort of special that requires ticket purchases you will not be abel to take advantage of them...
for example...
they may offer free dining again... this requires you to purchase at least a 1 day admission which will negate whatever you save by purchasing eary.
another offer that Disney has come out with is free ticket upgrade to park hopper and plus options... if you already purchased tickets with these options you are again out of luck... even if you did not purchase these options you may not be able to apply the upgrade to previously purchased tickets.
This is what is stopping me from buying tickets for our trip next year. If
ticket prices go up say 5%, for our family of four that might come out to an extra $40 or so. Free upgrades to the tickets would save us about $300 and free dining would save more.
There is also no guarantee that any specials/discounts will be available for when you are planning your trip. For the last 2 years, from what I remember, there has been no special offered during the time period from just after Easter to Memorial day.
Is is worth saving about $10 - $15 per person now as compared to a chance of saving $50 per person next year.