I'm relatively sure that you lock in the dining plan structure based on your check-in date. So, if they were to remove a snack from DxDDP starting in Jan (note, do NOT take this the wrong way... this is not being mumbled about, this is not fact, this is not even at the rumor stage!), you'd still receive 2 snacks per night if your check in was before Jan 1.
I do know that it does work the same for pricing as well as the other "based on check-in" things with the dining plans.