The Doctor will regenerate on Christmas Day in Doctor Who's annual holiday special, and the fans aren't the only sad pandas losing Eleven.
Clara Oswald (Jenna Coleman) will be the first companion since Billie Piper's Rose to have to get used to a new Time Lord face. (Friendly reminder that the Tenth Doctor dies alone. Happy holidays!) When Christopher Eccleston morphs into David Tennant in the first modern series finale, she is, to put it mildly, skeptical. Even when he proves to her that he is "the same man, always," she isn't sure he'd still want her along. But Clara's got an advantage. Perhaps she won't be as blindsided as her blond predecessor.
Don't forget that in "The Name of the Doctor", Clara jumps into the Doctor's timestream to save him from The Great Intelligence. Over, and over, and over again. In all of his incarnations. We know that Oswin the souffle baker and Clara the governess don't have any memory of this, but at least modern-day Clara has some understanding of the regeneration concept.
And then there was the 50th Anniversary special, where Clara comes face-to-face with three Doctors at once. She's pretty cool with the whole thing, though clearly Eleven is the Doctor she considers to be hers. She treats the War Doctor (John Hurt) like a friendly uncle and Ten like a particularly flirty friend visiting from out of town. Coleman and Matt Smith have settled in to their chemistry as believable best friends. Whatever Time Lord physiology knowledge Clara has up on her fellow companions, she is still losing someone.
On the other hand, we can't wait to see how the interplay between Clara and our new Doctor, Peter Capaldi develops. The reason that Doctor Who is still going 50 years strong is that it continually, by its very nature, reinvents itself. Clara, like the rest of us, will just have to roll with the punches.