it never changed, p in 1080p always referred to progressive lines as it came from movies/tv, 4k always meant 4096 at horizontal. We still have 2160p which is 3840×2160 They never replaced each other, just different definitions for screen sizes

It changed with the “K”. The marketing teams decided that 4K sounded better than 2160p. This is all the more ridiculous as 4K usually means UHD where neither axis has 4000 pixels ( 3840 x 2160 ). In the industry I am in, this would be called 8 MP ( Megapixel ) instead.

After the popularity of 4K, somebody noticed that they could start calling 1080p 2K to make it sound better than 1080p. If you say 3 MP, does that sound even better? I mean, 3,000,000 is more then 2000 right?