As suggested by Emily at thingsiatethatilove:
Plötzlichfreundersetzung: The confusion and anger engendered when a favorite TV character is suddenly and unceremoniously played by a totally different actor, while everyone else on the show goes about business as usual, pretending not to notice. Especially potent and damaging in young people, for whom this phenomenon can lead to lifelong trust issues, fears of abandonment, and an inability to believe one’s own perceptions.
Example #1: The classic instance of this is of course, Becky Conner from “Roseanne.” Seemingly overnight, Becky went from this person:

To this person:

For me, however, the true Ur-moment of Plötzlichfreundersetzung came when Victoria Newman from the Young and the Restless went from this:

To this:

Then, just as suddenly, back to this:

And now, all of a sudden, she is this???? WHO THE FUCK IS THIS PERSON?

She is not blonde, nor does she look like a displeased pumpkin, Victoria Newman’s two most defining characteristics! I watched the Young and the Restless for 23 years, which I am not nearly ashamed enough about. I loved the old, blonde, angry squashface Victoria, and I will accept no substitutions.
Of course, one could go further, and reference the most storied and ancient example of soap opera Plötzlichfreundersetzung, when beloved Denver-area sadsack and suggestible homosexual Steven Carrington transformed from this:

To this:

HOWEVER!!! Sir Aaron Spelling (knighted by Her Majesty Queen Rachel I of Shukertonia in her annual Birthday Honours) is not a egregious practitioner of Plötzlichfreundersetzung, as he actually bothered to provide the audience with a reason for Steven looking like a completely different person: an oil rig explosion off the coast of Indonesia that burned his entire face off. In fact, Steven in the hospital under and alias, and is so deformed as to be unidentifiable in his injured state, until his wily Singaporean plastic surgeon (who also played David Lo Pen in Big Trouble in Little China) puts two and two together, uttering the immortal line: “He says his name is Reynolds, but his belt buckle says otherwise.”
His Lordship Spelling shows similar care with the later replacement of Fallon Carrington, when Pamela Sue Martin:

transmogrifies into Emma Sams:

Resurfacing after being believed dead in a car crash, Fallon, we are told, was suffering from severe amnesia, and living under the name “Randall Adams,” an elegant solution that both explains her somewhat changed appearance (and newly acquired British accent) and allows the audience to live with delicious ambiguity. Is this woman in fact, the “real” Fallon Carrington? Will the “real” Fallon ever resurface to dethrone the imposter? And who is to say who is “real” anyway? Only a master could inject a plotline this patently absurd with such a metaphysical existentialist dilemma. Plötzlichfreundersetzung, or rather its inverse, at its finest.