Notice that
each dial has 10 settings, and the examples whose labels are shown have the labels in a consistent alphabetical order around the dial.
This suggests
numbering each dial 0..9 in that order and looking at the digits.
The Christmas settings are
0 3 0 8 1 8
0 9 1 9 2 0
0 9 0 1 1 4
and the New Year settings are
1 4 0 5
2 3 2 5
0 5 0 1
1 8
Hmm,
the left-hand ones in each pair are always 0,1,2. Maybe these are 2-digit numbers. Interpreting as A1Z26 gives us
C H R
I S T
I A N
and
N E
W Y
E A
R
suggesting that the person who changed the settings
replaced CHRISTMAS with CHRISTIAN. Perhaps they're very religious. Or perhaps the settings always have to form a word, and this was their best way of getting the specific changes they want. Having the settings at CHRISTMAS would have had (from M) coal for Black Hat and Santa entering through the chimney, and (from S) Santa using a sleigh.
So an obvious guess is that
the change was made by Black Hat, who is hoping for something other than coal this Christmas despite being (as shown in many xkcd comics) a very naughty boy indeed
and OP has confirmed that this is the intended solution. But there's another that I personally like better!
The dial's set to give Beret coal in his stocking. As we have seen at https://xkcd.com/1282/, Beret has some unusual preferences and sometimes prefers what's meant to be the booby prize. Perhaps the mysterious meddler is Beret, who really likes coal.