After a couple of appearances in the kid-centric Who spin-off already - and a cameo in "Journey's End," the fourth season finale of Who itself - K-9 will be appearing in a larger role when Sarah Jane returns later this year, according to producer Nikki Smith:
We've already got ten scripts at first draft stage, and I can now announce that K-9 is going to make a return to Sarah Jane's attic. He's been locked away in that safe, saving the world from a Black Hole, for far too long now... Right now, we estimate he'll appear in approximately six of the 12 episodes [of the new season].
With this sort of attention, it feels like there's only one way for K-9's star to keep rising: He has to become the new companion for the fifth season of Doctor Who. After all, we need a tin dog to replace the definitely-not-returning Mickey.
K-9 rejoins 'Sarah Jane Adventures' [Digital Spy]