of course it's too late to change it now! I'm sure the massive popularity of Sally Sparrow has been noted though.
I think CT will be fine, and add a very different feel to the dynamic or the leading pair so it isn't just a retread of the first three series.
I didn't like the Christmas episode, but that's for a lot of reasons (unimpressive static spider for one, not scary - spiders are scary because they can move fast and unpredictably, not because they stand in the corner of the room hissing, dribbling and waving their front legs about.)
I think on a more regular footing, CT will help it get away from some of the more run-of-the-mill romantic blah.