Thanks quisling. I've spotted one grid error and one error in my counting which brings my 5 and 9 letter names to 2 and 6 changes, so you've helped! My destination needs one further change which produces one real word in the right place but leaves 30d still nonreal.
I have utterly failed to find a set of changes for the 7-letter which leaves real words in their wake. I'm presuming the final instruction means that the letters involved can't have been changed at a previous stage.