S_pugh et al, you have touched on the issue that has been troubling me for ages. After changing 14d, 24 is still a word, as you say, ditto 20d/4a. I am not convinced by the loophole. Firstly, if any changes are not forced by the fact of the new entry being a non-word, the whole process is pointless. Secondly, the third sentence of the preamble says, "This in turn triggers changing another letter in the crossing (across) entry to make it a real word, and so on." The main part of that sentence refers to the third change in the sequence (to 22a); the phrase, "and so on" must apply to all subsequent entries, including across ones like 22a.
If any changes are strictly unnecessary then the preamble is not just ambiguous, it's wrong.
As I said earlier, Pointer sets traps, and he may have set an elephant trap here.