Hello, one of the maintainers of the wiki here. The major changes are generally divided into:

K2-K4

K5-K6

K7-K9

From my knowledge. Each group has the same general primitives and syntax, and major changes happen between those eras. Since K's design allows removal and addition of features, this is very hard to document. This, combined with Arthur Whitney's rocky relationship with documentation makes most sources of changes anecdotal. Most changes to primitives have been documented in the wiki's primitive pages. I'll have to summarize them in the history pages.

K4 has good docs, and K6 has a well-written manual on oK's repository. The rest are a fill-in-the-blanks game which have to be fished from archive pages and a lot of trial and error. K9, the latest one is still undergoing changes, but there are ongoing efforts to make tutorials in kparc/kcc and estradajke/k9-simples on github.