GSN's put out their schedule for that week, and not only is Millionaire going in to the 3pm hour, but overnight has been cleared out from 1a-4a, and is as follows (at least for the week of 12/18)
1am: Match Game x2 (different episodes from the two runs earlier in the day)
2am: Card Sharks (Eubanks) (no episode data)
3am: What's My Line
3:30a: I've Got a Secret.
The latter two have appeared for the last two weeks of December every year since 2009, so presumably it's a temporary move and not sure what GSN's plans for the 3a hour are for January - and whether the Match Game and Card Sharks hours are equally temporary or permanent is a question mark as well.