My experience (Dutch-born student in NL) is that "k" sounds sort-of less interested than "ok" or any longer variety, even in spoken Dutch ("k" is actually a used word in my environment). It seems that "k" corresponds to "fine"/"acknowledged" while things like "ok" or "oke" (yes my friends spell it like that, probably being too lazy to type é) correspond to "yes"/"agreed"/"acknowledged". I wrote "acknowledged" in both categories because they both acknowledge something, and sometimes the meanings do get muddled. :)

I don't see any change in my env, people still say "okee" in text to me. Sometimes "ok", but never "okay" (the English one)