The article's description of the new method is pretty far in, so here it is:

> Previous efforts to identify changing-look quasars have relied on variability in a wide range of wavelengths—a technique called photometric variability, which is known to miss lower luminosity quasars. The Bath researchers used spectroscopic data to assess the changes in very small wavelength ranges, allowing them to detect changing-look quasars that had been missed by photometric searches.

That is just brilliant! Although I do wonder how they deal with noise because I remember in my undergrad lab classes just how difficult it is to do low-light spectroscopy.

So… are they actually rare or were they just hard to spot?

still rare, but now easier to spot :)