Jogamel, the extra words are in 4, 6, 9, 21, 29 and 31.
I twigged what was going on with this and agree with Malone that it's a fun puzzle.
However, the preamble is a bit awkward. Think it should say something like: one letter in each of the six solutions (without keyword) is altered to form a new word for grid entry, with the six letters replaced, in clue order, forming the two three-letter keywords ? Not easy to explain but the examples given make it confusing to my mind.