Something seems very strange here.
There is no SK-operated TA flights on 24DEC and 25DEC. But there is plenty of availability on SK925 or SK943 on 23DEC (even in K) as well as SK8725 and SK8807 on IAD/ORD-LAS (as low as H).
Why would they move you to the 21st when they can accomodate you both the 22nd and 23rd?
SK2551/SK3909/SK8805 on 21DEC have plenty availability in most classes (down to Q on SK3909) so why would they put you in Premium Economy? Nice gesture? We are talking about airlines here...
If they can put you on UA/US coded flights there is quite a lot of availability on the 25th and the 24th.
I would check this with US as soon as possible.