I've completed the initial grid and have the theme, and have created 5 of the 6 new entries happily enough but for the one remaining I have multiple letters to change and I'm not entirely sure I've got the right answer. My new words are symmetrical, but the exact location of the changed letters is not.
I have another option where the changed letters
are symmetrical but in the context of the theme this other word usually has an extra letter so this seems dubious.