The TVTropes Trope Finder is where you can come to ask questions like "Do we have this one?" and "What's the trope about...?" Trying to rediscover a long lost show or other medium but need a little help? Head to Media Finder and try your luck there. Want to propose a new trope? You should be over at You Know, That Thing Where.
The closest trope that fits what you're looking for is Paranormal Episode, if it's only for a single episode, or a single chapter.
Otherwise, plain 'ol Genre Shift should do.
Something Completely Different for the supertrope, see also Out-of-Genre Experience
Early-Installment Weirdness and Growing the Beard are when this is part of a serial work finding it's groove, which often involves the evolution of a Myth Arc through gradual Continuity Creep
Cerebus Syndrome is when the work not only expands in scope but also grows more serious.
Edited by Scorpion451
This would probably fall under YMMV. A normal slice-of-life story suddenly and jarringly injects fantasy and/or sci-fi elements completely out of nowhere. I've mostly seen this happen in furry webcomics, where it almost gives the impression that the author got bored with their own creation. It's mostly normal slice-of-life for the first few hundred pages or so, but then one day there's alternate timelines, epic anime fights, alien invasions, gods and magic, time travel, and whatever else the author can throw in there.
I'd describe it as a Big-Lipped Alligator Moment that turns into a complete Genre Shift, if that makes sense.