Not sure why they changed their format from getting past contestants to getting new ones.
Perhaps it's too much to ask the current quarter-finalists to also do a Christmas Special with past celebs, but it might work if the current contestants DON'T participate in the Christmas Special, and they just get previous celebs on the show. I enjoyed seeing Colin Jackson, Derren Gough (though I don't really like his dancing generally) and Zoe Ball doing the Christmas Special. So I would like past contestants coming back again, like Kara, Chelsee, Ricky, Jodie, etc..
If they can get Katya back to do a Christmas Special, I am sure they will be able to get past pros back to partner those celebs.